In the school of fine magic, all magic teachers must undergo a course on forbidden dark magic such as witchcraft and voodoo curses and achieve a satisfactory result in the exam before they are qualified as a teacher. It is forbidden so they can only do theory but they must also swear never to teach their students anything on dark magic. Why bother getting into dark magic when you can neither use it nor talk about it?
This is quite simple... Teachers of magic are taught about the forbidden dark magics so that they will recognise when a student asks about or practises dark magic.
Every teacher needs to be able to recognise when a student is starting to head into forbidden territory, or worse, has strayed into the forbidden. How else would they be able to know when to discipline the students or deflect their questions that threaten to stray into forbidden territory?
To counter a danger you must first know its nature i.e. what it is, what it does and how to recognize it. Only then, knowing that, can you fight it.
So all mages are taught to at least detect and recognize black magic, including what traces it leaves when cast (both on the person and the place) and what counter spells are effective (or at least what basic counter spells are anyway).
This in turn means any mages deemed experienced and reliable enough to become teachers have to be exposed to dark magic, because otherwise they cannot teach others the basics of detecting black magic and protecting yourself and others from it. That said this 'exposure' is only done under strictly controlled circumstances by the most experienced/senior mages using dark tomes that have been seized over the years and then carefully secured out of harms way.
So the course covers the following;
- History of dark magic, its theory, symbology and tools (no dark tome required)
- Exposure to dark magic and its traces, (what it feels like). Tome required.
- Spell Types (Examples explained and demonstrated). Tome required.
- Detection of DM and counters to DM. This includes not only white magic spells but also common dark magic spells that DM practitioners might use to detect, attack or counter white magic practitioners. So basically both types of magic at the more common level. Plus importantly spells black mages can use to try and avoid detection (and how to counter those). Tome required.
Passing the course does not grant access to a dark tome. Only the most elite agents of the Guild or whatever tasked with policing dark magic get advanced training in all aspects of dark magic and full access to dark spells. And even then only under supervision and as required for specific missions.
So-called "dark" magic is really just the fundamentals.
When you're learning how magic works, you need to learn how to shape and channel flows of woo. Woo flows through everything, so to learn how to affect anything you need to learn the rules for how it flows and how to affect it.
From there, there are some easy and obvious shortcuts, like killing things to use all their woo. The problem isn't that the "dark" arts are inherently dark knowledge, it's that direct application of any of that knowledge is evil. Most of the remaining curriculum is about how to avoid manipulating woo. That also lets you have white magic as the super-advanced learning that lets you manipulate woo in helpful, healing ways (like the difference between a gladiator and a surgeon: both know enough anatomy to put a blade in the right place, but the outcomes are fairly different. )
Because you need to be ready to counter people who do use it and don't have your scruples or common sense
This isn't specifically for teachers, it's just higher-level magic. Consider it degree level, or black-belt level. Most people with degrees or black belts don't teach - but getting that degree or black belt is naturally a prerequisite for teaching.
I used to do a little jiu jitsu. Part of jiu jitsu is learning how to apply chokes, and as a student you also learn to tap out when you're in a choke and can't escape. But the brown and black belts occasionally practised fully choking each other out, because at that level you're working on skills beyond just what happens in the dojo, and your average pissed-up Friday night brawler isn't going to tap out nicely or let you tap out. The person applying the choke learns how to actually make it effective, how long you have to hold it on for, how to tell the person is out, and the first aid required to keep them safe when they're out. Meantime the person being choked learns how to process this mentally so that they don't panic and can respond when someone grabs them from behind in a pub car park.
The same applies here too, especially if your worldbuilding allows for combat magic. (And really, who's going to create a fantasy world with magic and not have magic duels?) The first steps in teaching anything dangerous are always very carefully judged so that the trainee doesn't seriously endanger themselves or other people, whether that's in woodworking, tree surgery, hang gliding, scuba diving, weapons skills, or anything else. Until the student can show they're working safely in a sandbox environment, they aren't ready to be let out.
And once they get to that level, they're ready to deal with the rest of what the real world can throw at them. Their basics are solid, but more than that, they've been drilled with a respect for how they can hurt themselves or other people if they screw up.
It doesn't mean people won't still dick around - drivers speed all the time in spite of being taught the dangers, for example. It's illegal, and you know you might be caught and fined, but it's mostly harmless and a good driver knows when it's safe. Mild witchcraft is likely to be the same - if you've got a problem with weeds in your garden, why not hex the plant so it dies, for instance. But if you accidentally hex the farmer's field next door, prepare for a big fine. So your magic users also need to be ready for clean-up operations when people don't use common sense and dick around without adequate skills. Hanlon's Razor still applies: Never attribute to malice that which is adequately explained by stupidity. Or more to the point, whatever someone could do intentionally, a suitably incompetent person could also do just by being a bit crap.
As with other crimes, there's a difference between minor accidents happening and major injuries or deaths. We already have many ways people can deliberately or accidentally kill themselves and other people. Magic merely adds another way to that long list.
How can you test bullet proof armor without also creating guns and bullets?
To use Harry Potter as an example, there is the Dark Arts as a subject. Defence against the Dark Arts.
One of the teachers demonstrates the 3 unforgivable curses in class. One of which is, basically, mind control. The Defense against it is having a strong will. How can you know how to defend against it without knowing about it and, possibly, practicing against it?
There are weapons we have that we'd never realistically use against each other. We have schools and scientists creating defenses against those weapons "just in case". How can you create defenses without learning about, researching and even creating mock - or real - weapons to defend against? How can you create a vaccine against a bug that doesn't exist? Bullet proof armor without having bullets?
So TL;DR: You have to know and use within controlled and safe circumstances the tools and weapons against which you have to protect against.
Magic is analogous to a computer system, and each spell is a program. Beginners know only how to use spells made by others, and beginner-level spells have a myriad of safeguards built in. More trusted spellcasters are taught spells without as many safeguards, and masters can write new spells. What the general public describes as "forbidden dark magic" are merely spells with no safeguards built in. Any master can make these "forbidden dark" spells, simply by leaving out the safeguards altogether.
Consider your basic "Cut Bread" spell. It functions by destroying all matter within a thin flat region, but has safeguards to ensure that it is only being used on bread. Those safeguards verify that the target region only contains bread, and shrinks the target region as necessary to avoid harming anybody.
The combat spell "Blade" operates on the same principle as "Cut Bread", but without those safeguards. It continually destroys solid matter within a narrow region, which may includes somebody else's body. It's a more dangerous spell, but is also much more efficient as it doesn't need to check the safeguards.
Master spellcasters can take this even further. Even "Blade" has some safeguards, to restrict the region of destruction and to avoid harming the operator. A master spellcaster could create a generic "Destruction" spell, with no safeguards whatsoever, which is limited only by the energy provided to the spell. These are the forbidden dark spells, which every master knows how to make, and every master knows not to.
Dark magic requires you to tap dangerous forces and contract with deadly beings.
It's not just a matter of casting an evil spell. Curses tend to involve binding an evil being to a person to hurt them, and that requires negotiation with very dangerous and hostile magical forces that can hurt society.
Knowing the theory helps but it's not a very small first step. Actual dark magic requires contacting these beings and negotiating a price. This contact means a lot of higher level spell crafting with kinder and better entities is now impossible.
If you start casting dark magic, you won't be able to cast the super epic light magic that requires negotiation with good entities.
As such, more powerful people have less incentive to go evil. They'd lose their most powerful powers, and it would require months or decades of negotiation to gain the same level of faction relationships with the dark side.
Do you want to be able to cast a boils on the penis of your enemy dark spell, or a spell that will resurrect a friend light spell? A spell to make your enemies have mildly bad luck, or a spell that lets you burn a village down with holy fire?
A student can go dark, and they'll lose nothing, except their lives when they turn evil. Teachers lose more. Teaching teachers the theory helps them keep student on the light side.
Dark magic is the exploitation of mistakes - teachers must learn about it to teach students which mistakes are particularly dangerous
A dark magic user isn't actually much of a threat to people who use no magic- instead their power lies in the ability to corrupt the spells of others to do them harm.
Essentially, magic is hard: everyone makes small mistakes from time to time, and these mistakes cause your spell to do something that you didn't want, where the exact outcome initally seems random. For example, a student messes up when transforming a teacup into a cat, and the cat comes out with pink ears!
However, the unwanted outcomes aren't actually random: they depend on the exact magical circumstances where the spell is miscast, and these circumstances can be subtly manipulated.
Dark magic users learn to exploit these mistakes to make your spell do their bidding- instead of the cat just coming out with pink ears, this allows everything from stealing your thoughts to stealing your powers!
As a result, it is crucial even for a beginner to learn the kinds of mistakes that are exploitable by dark magic users, so that they can be much more careful about them. In particular, if a student makes a charm they intend to wear in public, it is wise to first show it to a trusted teacher who can check it for mistakes that would make the student vulnerable to any dark mage they come across.
(All of the above is true in our world if you replace magic with programming and dark magic with hacking. Real world computer science curricula have to make difficult choices about how to teach computer security while convincing students not to go practice their new skills against public websites)
Dark Side is Built in
Understanding Magic as a system requires a good knowledge of both white and dark magic. Studying dark magic is more than a purely defensive act, instead, understanding both sides, including the darkness is vital. As you complete your education, and go on to your professional calling as a sorcerer, watch out.
At the elementary level, it is known that a spell controls both a negative and a positive force like magnetism or electricity. At an intermediate level of magic casting, you may learn attack spells which require negative emotions, and it may take concentration not to let the take over. Human fight or flight mechanisms may be required for an attack spell, but also can push a spell out of control. Depressed or bitter intermediate magic users are dangerous. It gets worse. At an advanced level of magic, the power that is accumulated increases the temptation of going to the dark side and become a world eating super villain. "Absolute power corrupts absolutely."
The teachers have responsibility to keep the students safe, and also come up with good lesson plans that show the risks in each branch of magic.
The fine print:
Divination: Divination requires understanding of future problems so it has a dark side. Wanting to know the future on a low level may not have a big dark side, will there be good food in the meal hall tonight? beans. but consulting the fates on future problems can lead to visions of extreme darkness. *What is the worse thing that will happen next year? demons will attack the school and possess the head teacher and you will die in the battle.
Abjuration: Although it is defensive, magical barriers can pose dark problems, such as locking beings in magical prisons, or depriving people of their magic powers just when they really need them.
Enchantment: Tempting to give an artifact too much power.
Necromancy: Reviving the dead party members might be useful in certain situations but creating Liches and Undead armies can pose problems. Enough said.
Illusion: Sounds harmless, but illusions can blur the difference between reality and perception, such as creating a dense carpet of spiders or snakes - a gateway to madness.
Conjuration: When some giant demon arrives with a proposal to lose your soul it is probably a result of a conjuration.
Evocation: Evoking guardian angels sounds OK. If evoking Tidal waves, Infernos, Rolling thunder, or Whirlwinds, there are lots of possibilities for things going wrong.
Transmutation: Altering somebody into a goat or a frog for an hour sounds OK, but for longer it might be a problem.
Forbidden Dark Magic Drills
First, let's hopefully try to lighten the mood a bit, because this is about to get dark.
Forbidden Dark Magic often requires specific materials in their rituals, or if cast as a spell, you need to use a wand - can't cast a forbidden spell just by speaking its name - you need wand movements and an actual wand. So when practicing the basics as a teacher to get familiar with what to spot, you simply don't use those materials needed, or make sure your wand is safely stored away; worst cast, you end up avada kedavra'ing your desk drawer.
So if you use scented candles (Or if those work as well, glowsticks) during a summoning ritual, it doesn't work, or if you use a lead bar as your casting implement, it doesn't actually cast magic out the far end. Teachers can learn what doesn't work, and practice a bit both to be able to spot others trying to practice the forbidden arts, but for additional, potentially darker reason.
So, what happens if Forbidden Dark Magic is more commonplace than preferable?
A lobbying group in the magic community for the manufacturers of the materials needed for the forbidden rituals, and wandmakers, doubles down on the belief that their materials are not at fault. "Candles are a perfectly fine light source as long you're handling them safely" and "The only thing that stops a bad guy with a wand is a good guy with a wand" become known phrases among political groups in the Ministry of Magic in charge of writing the regulations on said materials.
Fire Drills aren't just regular drills in the Magic world - teachers are tasked with creating fake summoning ritual areas with scented candles or glowsticks out of sight, and if no student comes across them and repots them - or takes them down, they end up starting a controlled burn throughout the school, as kids learn to single file and exit the building, or cast fire extinguishing spells on reflex.
The main reason for the practical approach? Sometimes, actually Forbidden Dark Magic is performed at school by kids as a prank or by accident, and having to handle actual outbreaks of ritual summoning turns out to be a known source of danger.
As would Forbidden Dark Magic spells...sometimes there are actual Forbidden Dark Magic users on school grounds causing a whole lot of danger. So teachers are required to practice the timing of forbidden magic and incantations with lead rods instead of their spell casting implements so that they can simulate fake users, to teach their kids to learn the Unforbidden Lighter Magic spells that provide protection against these spells, or how to dodge said spells based on the reflexed of a teacher who's practiced and has reflexes prepared such that, if you can dodge the line of sight for a teacher using a Dark Forbidden Magic spell in your direction, you can dodge it in real life - or learn to take down the caster in a non-Dark Forbidden Magic way, to stop them from being able to cast the spell.
To that end, teachers need to know where the limitations are, specifically to prevent themselves from actually putting kids at harm - as well as potentially having practiced against lead bar targets so that they can evaluate if students would have actually gotten out of the way of someone casting Dark Forbidden Magic in their direction.
Maybe the only (or best) way to teach Magic is by controlling a student’s mind, guiding them through the use of Magic.
Similar to a driving school car or training aircraft with two sets of controls where the teacher can always take over if things get dangerous. Except in this case the teacher sits in your head instead of the co-pilot seat.