Yep. It was on work experience with my dad in a London office (I must have been about 14 or 15) and there was this really fit New Zealander who I was working quite close to. Eventually I just couldn't resist and went to the toilets to have a wank. It was quite a private toilet thought (One you could lock the door on, not a cubical type place) so it wasn't so bad.
I masturbated in school before. I mean in class. I was just bored and wanted to see if anyone would even notice. Nobody did.
Did you check to see if anyone in next period noticed the slime under the desk? When I was a cop I once jerked off in a nasty toilet while walking my beat on a deserted wharf on night shift. Of course that was more private than trying to be discreet in a latrine stall in a barracks with other guys coming in and out. But you had to do it somewhere or you would explode.