I knew a guy in school that always used to do it, I never witnessed it but I knew it went on and I bumped into an old teacher at work in the summer and she was remembering when someone (who later started playing professional football before going to prison for rape) jacked off bang infront of her. She also recalled when the guy that jerked off all the time did it in a condom and threw it at the whiteboard.
I once got off of gym class to finish my art portfolio and was put in an unused room because there was a class on. I hardly did any work because I ended up jacking off, and I recall doing it a couple of times at college when I was 16/17/18 when we were allowed in the rooms without supervision.
I seem to think I've done it at uni as well, but other than on my friends floor when I had passed out drunk and he was still asleep in the morning I can't think of where or why I would have managed it.
I also used to jack off in the office when I was on work experience when I was 15 because they left me alone every day for hours.