My old physics class had a hugely raised floor, full of toxic
dust and stuff. Now, before I go on you should all know that I was our school's physics
Übergeek, and in fact
knew more physics stuff than our teacher (poor woman, she had me two years in a row, and often as not I'd end up teaching the class esoteric tricks to make the syllabus easier), so she was kinda used to me playing up.
One period, arriving before anyone else, I discovered a trapdoor in the floor. It had been under my chair all term, but I hadn't really seen it before. So, I hid under the floor. The toxic dust mutated me horribly and itched my eyes, and after the period had been going for a few minutes, I lifted the trapdoor (at this point Fran, the german exchange student (the cute female one, not the hairy male freak who was a David Hasslehof fan. Got that? He really is big in germany) actually screamed a little), rose out of it to about the waist, and then stood in the floorspace while taking my notes for the rest of the period... people noticed one by one and laughed a little, but mainly, I had fun...
Not as good as the time we hid in the ceiling in english though.