I climbed all the way to the top of the clock tower (outside) at my old school and tied some massive inflatable pigs up there.
It was savagely high up, not sure how they got them down, but they definitely wouldn't have had a ladder that would have been up to that task.
The other ones that I'm aware of in my school were pretty much vandalism, but some were still amusing. We had a teacher there called Mr Pantridge and someone wrote something on the roof of the Maths block in HUGE letters. It didn't say "Cook Past Babtridge", can't write what it said, but any Alan Partridge fans will be able to work it out. Was in enourmous letters and could be seen from the science block very clearly.