Several years ago, in a school district not very far away, we had a gnome. The gnome got planted in different places around the school. The rule was that if you found a gnome in your room you needed to plant that gnome in someone else's classroom undetected.
This prank was started anonymously. Just a gnome and a printed note.
It was slow going at first, but within a few weeks the gnome ended up in some very creative places: in the trophy case, standing in the window above the door to the English class, and my favorite: inside a bass drum looking out.
After weeks of suspicion around the staff, the usual subjects (the guys) had been questioned and no one could figure out the rampant prankster.
No one expects the little Spanish teacher!