I heard this story that supposedly happened years ago at my HS. Some seniors brought 3 pigs and let them loose at different places in the school - yes INSIDE the school! They had big numbers painted on the sides of the pigs: 1, 2, and 4. After the pigs had been caught, there was a big ruckus as the admins desperately searched for #3. Genius!
edit: Forgot. My own class's senior prank was to build a wall out of concrete blocks across one of the busiest hallways of the school. Took them forever to take it down.