Yes, sure, when you are administering even OTC substances directly to people who don't consent, that's clearly criminal.

But the student putting a legal amount of laxative in the teacher's coffee is distinguishable from adding it to something you own which you suspect, but do not know, may end up stolen and consumed.

We're really arguing the core legal issues of actus reus (adding a legal amount to your own food, and the consumption of the food itself) and mens rea (whether you intend to consume it or not, whether you intend for the other person to consume it, or whether you are neutral). But I think there are very plausible defenses here for both elements, even if I can see how either verdict/judgment is possible. That's why it's an interesting fact pattern.