This one time.
But other than that no. But I know of a few kids in my school who did. If you don't want to imagine anything containing someone's mouth bleeding badly, turn back now.
These boys at my school were in a stereotypical fight, ie. Kids crowding, them in the middle, stuff like that.
Anyways, the smaller one was biting on his lip, and the other one went and punched his mouth. So, his lip was bleeding. And I mean BLEEDING. His teeth were in his lip, and he had to go to the hospital.
Stupid of the guy who punched him, eh?