We had Shitler. Someone kept defecating in the boys washroom and drawing swastikas on the walls with it. The staff couldn't figure out who it was, and started closing down washrooms. At one point there were only 1 or 2 men's washrooms open - and Shitler still struck again. No one ever found out who it was.
In my junior high they had a mysterious urinal pooper. They spent months trying to catch the culprit. They even resorted to a $25 reward and a little campaign in the morning announcements. Then it turned out to be a handicap student who couldn't bend his knees. The posters and campaign quietly disappeared without a word.