If any students read this, I do not condone this sort of behaviour, you'll receive detention for the rest of your life if you try it, and so on.
That being said, a bloke I was in training with, we're fairly sure he must've been hung over, which was an awfully stupid move. He came to training in his robes, and we were working on physical defence and ground fighting, which is difficult with robes on, so we usually learned in sweats or other sorts of clothing. I was his partner, the only girl in class, and time came to disrobe, and what do you know, nothing underneath. He was just as surprised as we were, though likely not for exactly the same reasons.