I've always been a fan of circulating fake newsletters, memos, bulletins, etc. If the school has a daily bulletin of some sort, target that.
Steer clear of personal attacks and insults. Those along with destruction of property are what bring consequences. In high school, we circulated a fake bulletin with a very confusing schedule of meetings and assemblies, as well as an even more chaotic breakdown of who had to go where. Some people thought it was a spoof, but others took it seriously. The best part was that a couple of administrators thought it was funny so nothing happened to us.
I'd recommend fake meetings, announcing a change in hours for popular things, maybe "close" a department or building because of budget cutbacks.