This is a very silly question. :'-) I will do my best. I'm looking at a few articles about why people start smoking in general.
If it's a society-wide thing were most wizarding teenagers in general smoke, you could plausibly suggest that it's a cultural phenomenon related to a supposed medical benefit, like in the United States in the 1940s and early '50s. Considering that the wizarding world doesn't necessarily have a big grip on basic science, including medical science, that might make sense. It might also be possible within the wizarding world that the health risks are mitigated by other factors, making things like nicotine's anti-anxiety effects outweigh things like cancers. In wizarding schools where Muggle-born students come in and know the effects of smoking, it could be acts of rebellion and liberation from the new knowledge that any downsides of tobacco don't need to affect them.
As for why students would then need to hide smoking from teachers, though, maybe it's a new ban by a Muggle-born headmaster, and even the teachers have to start hiding their smoking. It might help if there's just a single person responsible, because then it would make sense that numbers are stacked against the anti-smoking population, like you want.
If it's just that a current school population smokes, then it would have to have a more singular locus of influence. Maybe a previous student who smoked left the school, or even died in some horrible tragedy, and in honor of them more students outside of that person's immediate friend group (who may already be heavy smokers) obtain cigarettes and end up getting addicted.