What material for weapons and tools would a tribal/medieval society use if there was a magnetic field preventing the use of iron? The iron would be attracted by the ground, obviously. It can`t be used for arrows for example as they would not get to far.
Ceramic knifes would be cool:
A ceramic knife is a knife designed with a ceramic blade typically made from zirconium dioxide (ZrO2; also known as zirconia). These knife blades are usually produced through the dry-pressing and firing of powdered zirconia using solid-state sintering.
I'm not sure if this technology is feasible for a medieval society - but its fiction, so why not?
Earth's history is filled with non-magnetic weapons and tools. The Iron Age was named for the increasing use of iron in tools and weapons -- before that, bronze, copper, wood and stone did the same jobs, perhaps not quite as well, but well enough to do the job for many centuries.
I used to work for a company that makes 1.5 T and 4 T MRI machines. We used brass tools in and around the magnets.
They did some research into an open magnet design to be used in the operating room during surgeries. The surgeons would have used brass instruments, with ceramic scalpel tips. However, the concept never made it into product.
It should be noted that in a magnetic field, torques are a bigger hazard than linear forces. In other words, you are more likely to have a tool twist your wrist than get "sucked" into the magnet.
Stick with bronze.
You may have heard of the bronze age, a time in which all metal tools were made out of bronze (not iron or steel). Bronze is a non-magnetic metal alloy used in human history to make tools, and weapons, and pretty much anything you would make out of iron.
Bronze is a simple historically friendly answer to a substance that can easily be used in a strong magnetic field.
After the bronze age, a civilization might proceed directly to the plastics age if iron is not feasible.
Use whatever materials you want:
As you say:
The iron would be attracted by the ground
The thing is that arrows already are attracted to the ground - by gravity. You have to ask just how strong this magnetic field must be to noticeably deflect arrows.
Some experiments have been done on this with bullets including by the Mythbusters but they used non-ferrous ammunition.
The following video however shows an experiment with a powerful neodymium magnet and a steel ball from a low-powered air rifle. There is some deflection but they fire very close to the magnet and I don't suppose your landscape is made of neodymium. https://youtu.be/pXDLGNKoR2c?t=279
Firing near a the world's most powerful MRI machine that has a 45-ton magnet and generates a 9.4-Tesla magnetic field would definitely deflect or even stop an arrow but there's nowhere to plug one in during medieval times.
Since there were many suggestions of non-magnetic metals: They aren't usable for many purposes, specially for fast moving parts: Eddy currents will slow down the motion.
So the culture should go for isolating materials: Wood, ceramics, and glass.