One of my mates had the Casio watch with the TV remote built in. We would take turns wearing it in French and making the channel change. One minute it would be the french video we would be watching and the next Playdays or Going for Gold would be on the TV (early 90s). This went on for some time the French teacher never realising what was causing it.
Then one week we were due to watch a video in History which was a rare treat, the watch came out and luckily I wasn't wearing it. The history video started and about 3 minutes in the TV switched off. History teacher put the TV back on and 3 minutes later the channel started changing to static. "RIGHT! WHO HAS THE REMOTE CONTROL?" the teacher shouted accusingly, he was far more savvy than the French teacher and we had made a complete underestimation. "PUT YOUR HANDS OUT ON THE DESK". He paced around pulling up sleeves and located the watch with ease. It was promptly confiscated till the end of the year. The lad who had it on was not the owner, he got detention but took the whole wrap and never dobbed anyone else in.
A brand new tin containing a roll of magnesium was opened for an experiment in Science, when the teacher went out of the room near the end of the lesson it got pocketed. There was a couple of meters on the roll and we had great fun lighting strips of it and getting up to all sorts of mayhem.
Tidying up after a physics lesson the teacher counted the mercury filled glass thermometers back in and realised there was one missing. It was the lesson before lunch and the teacher said "Right you better start looking for this thermometer because nobody is going to lunch till it is found". So we start checking high and low taking all the beakers and apparatus we had been using back out of the cupboards. After 10 minutes it hadn't been found but the teacher wasn't relenting. I was thinking one of the lads must have tried to pocket it. After another 15 minutes it still hadn't turned up and everyone is angry they are missing lunch. At that point I put my hand in my pocket and felt a thermometer! I must have just put it there in between doing parts of the experiment and writing up. I had no idea I had it and wondered if I should try and pretend to find it on the floor or something. But with 25 other people all searching I would need incredible slight of hand and a location that could be explained how it wasn't already found. So I just kept quiet and eventually the teacher let us go with 10 minutes of lunch left. I never told another soul.
We had a drama room with a a raised stage area at the back which you could crawl under. One week we went under it before the Drama teacher arrived and just stayed there for the whole lesson. The next week more people did it and we were inevitably rumbled and all got detention.
We made one of our French teachers cry by making noises every time she turned her back. "Mrrrrp", "Brrrreeee!", "Bllooop" just totally random and loads of us would take turns coming up with ever increasingly strange and annoying noises. She just lost it cried and said we were horrible, I felt pretty guilty even at the time seeing a grown woman sob. Feel absolutely terrible about it now. My wife is a teacher, kids are so cruel.
Can't really think of much else that interesting really.