This didn't happen to me, but it happened in my class.
In computing, we have supply for a few weeks while our normal teacher is in hospital, and he's nicer than our normal teacher, so some people in my class take advantage of the fact that he said he'd only give an after school detention to people if he had to physically drag them out of the classroom, by literally not doing any work, throwing paper aeroplanes, yelling across the room and moving seats. (This also happens in maths, but its just our clueless normal teacher.)
Both of these are top set.
Pls save me