I doubt they violated anything, I don't believe they can force you to open anything, sounds like they played you to me. If they get you to open a drawer, it' because they can't legally search it and they are tricking you to get around the fact they have no warrant.

Doesn't stop them from going and getting a warrant and searching whatever they want, or maybe getting campus security to do so, but it's not wise to ever open anything for a cop, your door, your car, your trunk, your backpack, anything... If they have a legal right to search they won't ask, they'll just open it.

Just politely say no, and answer no questions without a lawyer present, no questions at all. Respectfully and calmly say no.