Prank # 1 from me - Questions...questions...

While the teacher's talking, ask a question on something he/she was talking about 5 minutes ago. He can't get angry, because you're participating in class, except you will set him back. The rest of the class will be free while he re-explains the stuff.... If you work together, you could have people take turns doing this sort of thing, eventually the whole period will be gone. This works best with teachers that have a habit of straying off the point ;-)