There are schools where the guy and girl toilets are located side by side. These toilets then separated only by a wall, sharing common plumbing facilities like water pipes and WC stuff.
In the early 90's, (not sure abt now

), the ceiling are those white styrofoam square panels. Some guys I know will climb up these white panels, crawl over to the girl's toilet, slightly lifting it up and PEEP! Haha, I was told girls DO masterbate in school.
Remember there were porn videos where video cameras are hidden on ceiling panels? Well, these guys did the same thing. I hear this story directly from the guys who are doing this, so I'm pretty sure this is true.