I wouldn't bring the betta back to school under any circumstances unless it was with his full current set up on a temporary basis to teach proper betta care. I would be too concerned that an unknowing or cruel person would mess with it. You could always say it died or ran away. If they give you problems about it you could make it into a bigger issue for them: animal cruelty which is a problem I'm sure they don't want to deal with. A little life lesson, those people who are "in charge" want as little problems as necessary to deal with and will often try to avoid conflicts like this. ;) Good work.