When we were kids, as a joke we placed a white pastel crayon on the chalkboard ledge during a rain lunch day, and with no teachers in the class during lunch, we egged on another kid to draw male body parts on the board (with what he thought was chalk), and never to turn down a dare, he drew SEVERAL of them of all shapes and sizes.... well, when you try to erase a pastel chalk off a black board... IT DOESNT COME OFF!! lol.... water only makes things worse!!! When class started, and the teacher came in, lets just say she was NOT amused!
We had to get the janitor to use wild coyote ACME chemical solution to get it off!