At our school we come in on rollerskates/blades/boards and went round like that all day.
At another school they stapled and stuck the furniture to the roof during the night.
If you have an ICT room, swap all the keyboard cables around behind the computers so that the keyboard in front of one computer actually belongs to one next to it.
You could also bring in 3 toy sheep (or something of that nature), label them 1, 2 and 4, then the teachers will be running around all day trying to find number 3!
OR you could put red food dye, in a little bag with some holes in it, into the water compartments on the toilets (you know the place where water is stored before you flush, i dunno the name); make sure it's under the water level tho.
If your school is an all boys school you could go around town and put cards into the telephone boxes etc. with your school phone number and say 'Ring this number for hot girls'. BUT if you have girls in your school , DONT, cause it's kinda disgusting for girls.
Hope that helps!