I've done many stupid things in my time most of which I don't wish to disclose on a public forum.

As for school-related stuff...

One thing that immediately springs to mind is when I stuck a plastic ruler into a radiator during one French lesson. Usually, when you dropped a ruler down the vent it came out back out at the bottom. Not on this occasion. Initially I thought nothing of it, but when the fumes started to hit and people started to feel a little light-headed, I started to panic a little. Luckily, the teacher noticed and switched off the radiator. A molten plastic mess was the retrieved. Nobody saw me do it so ultimately I never got caught.