Our Biology teacher went to the staff room to nick us some biscuits, so we locked the door when she left. One of my friends then got the anatomy model (you know the waist up ones where you can remove the lungs etc...) put it on his stool, dressed it up in his blazer and tie (possibly shirt too, too long ago to remember) and hid. The teacher came back before we'd finished so we had to go unlock the door, making her suspicious, but she didn't notice right away, and when she did her face was hilarious.
Most of my biology GCSE was like this, the one thing I didn't appreciate was getting the pickled lizard put in my blazer pocket :|