if it's anything there like it is here, you should be fine. academic integrity is not to be taken lightly, which works in your favor when you are innocent. for someone to accuse you of it is HUGE, and it can definitely blow back on him. if he's just making a foolish and unfounded accusation, he will either realize what he is about to do to himself and back off, or be in a heap of trouble when they find out he's falsely accusing and has no proof. if for some reason he's doing it maliciously, again he has no proof and you have a room full of witnesses. i doubt this is the case, but if it is, he's in an even tighter spot than if he's just being an idiot. in either case, don't be timid. make it abundantly clear you will take this straight up to the dean if you have to. if you think this is going to turn serious, you might consider talking to the campus police. they can probably take statements from the other students and whatnot.