The nastiest one I've seen was someone stuff not in the toilet, ON the toilet.
You call that bad?
Broh, at my old school, people would have diarrhea all over the floor, and then do loving cave paintings on the wall with them.
Mind you, I went to a school full of rich stuck-up kids, no special ed, and nothing.
I stuff you not, there would be a puddle or pile of stuff in the middle of the floor, then the dump-ee would put their hands in it, and finger paint all over the walls and mirrors.