We had a history teacher who used to walk around the room talking about whatever the subject was with his eyes closed. It was always a good laugh when we all swapped seats as he was talking - every time he opened his eyes the baddies would be in a different place. He would always take his watch off and put it on his desk, every so often returning to pick it up and squint at it - until, of course, he discovered it had been sellotaped to the desk.
Incidentally, he was also our form master for most of the time I was at school, and we thought he was great.
I also once swopped the sign on the door of the headmaster's study with the one from the library, on the second day of a new year. 'The Boss' was inundated by new boys who had been told, naturally, that they should just walk straight in if they wanted anything from the library.
In't skool grand?