My contracts professor, who is not generally on the liberal end of things, blinked when the class mentioned the under-21 rule (obviously this didn't apply to anyone in law school, but it came up in the context of something or other), and said that as far as he heard it only prohibited buying the alcohol, and didn't say anything about whether you drank it. (Do not cite this if the police catch you (you-world, not you-Maya), without having done some research of your own; it is not his field of law and neither is he, or I, under 21. But he may be right.)

That said, I'm almost positive there would be some dorm/undergrad restrictions involved in at least some of these cases. And I think there are also rules about overage drinkers buying specifically for minors.