when I was in high school the senior class above us took everything out of our headmasters office,put a park bench in there, then got a goat. The goat shit everywhere.
For our Class We bought tons of plastics cup and filled up all the hallways with cups of water, that way they have to pick them up one by one.
Also I remember some class putting bird seed everywhere(On top of the buildings and all the walkways), it attracted a lot of birds and they pooped everywhere.