One time at a school near me, students let a sheep up the stairs into the building. The funny thing is, you see that sheep can climb up stairs, but they won't go down them. Can't remember how the got them out, possibly with a cherrypicker crane.
A neighbouring school stole the witches hats from the road construction outside their school on the main road/highway, and redirected traffic into their school onto the fields. They had to call in traffic controllers to help get huge trucks off their hockey field.
Once, many years ago at Sydney University, UNSW students went to our famous main quadrangle and burnt U N S W into the grass of our square. Don't do that, it's very destructive and mean.
In our final assembly, some kids taped mobile phones behind the curtains of our auditorium and rang them in the middle of speeches and such. The headmistress got pretty pissed off. Except the stupid girl got caught when she went to retrieve the phone after the assembly! Do this with alarm clocks taped under chairs.
Oh filling rooms with balloons is prettty fun too. All you need is a locked office with one window slightly ajar. Pump helium balloons through the window.
We also had a day where everyone brought their dogs to school. That was really fun.