The legal drinking age is 21. Rebbeim do NOT have a heter to ignore that. Parents do not appreciate when Rebbeim take it upon themselves to serve alcohol to their children and make the decision to do so without the permission nor opinion of the talmid’s parents. It is not up to the Rebbe to make that decision on his own or take that control away from the parent. And no, until the Law says the talmid is of age to make that decision on his own, he isn’t.
A Rebbe is supposed to be a good role model in all ways. Serving alcohol to talmidim who are NOT ALLOWED to drink is irresponsible at the very least.