You probably had to be there....
I was about 15 at the time, a complete shit at school and wanted to be out in the big, bad world instead of sat in classes having to listen to talentless teachers who hadn't really got a clue. Every day i was involved in some sort of tomfoolery which inevitably ended up with me in detention/having to visit the headmaster or spending the lesson sat outside.

We'd been out for lunch one day and were sat in the formroom before the tutor came in to regisiter us for afternoon lessons. This usually lasted about 20 mins before sloping off to other lessons. One lad, who wasn't in our class, had a stash of porn in his bag, which gave me a wonderful idea. We selected a lovely japanese centrefold, wearing just ice-skates (WTF) and attached it to the old-school revolving blackboard behind the teacher's desk. The board was revolved to hide the said image, hoping that she'd later discover it. Everyone was in on it and we thought it would be ace - she was a proper fuddy-duddy.

The tutor walks in and sits down and starts taking the register, straight after, she asks each one of us to go up one by one in alphabetical order to check our parents' contact details. We had not expected this turn of events, but the first person went up and edged the board down very discretely - i felt nervous, as this wasn't the plan. This went on for about 10 minutes with all of us doing our bit. The skates were poking out of the top so one of the lads thought he'd be a bit more brave and expose a bit more flesh.

The picture was alsmost directly behind the tutor's head at this point. We were all trying not to laugh and hoping she wouldn't stand up and turn around. My heart was beating as I knew everyone would grass me up.

Just to make it that bit sweeter an OFSTED inspector walked in and sat right at the back of class. None of us could do a thing and the tutor hadn't got a clue - I wouldn't have done it if I'd known an inspector would walk in. We all sat there trying to to laugh at the quim-tastic ice-skater. We got such a bollocking the next day, but it was worth it and surprisingly enough, no-one grassed me up. Bonus.
(madamebonsoir, Wed 25 Jul 2007, 16:47, Reply)