This would have to be used in a private school setting so people would sign a form and it would put a new meaning to the word "private" school. There would be a small rectangle for taking a dump. So the only form of sexual activity one could attempt is putting a finger in their ass, and if a guy got so desperate to masturbate that way, he wouldn't be able to hold an erection due to the tight metal against his shaft.
These things would also prevent things like staring at someones crotch or ass in a sexually gratifying manner because of the bulky metal.
These would be designed to be non-removable without the key. If a student fails to make all A's by the end of the year, the student would have to wear the belt throughout the summer too, in order to prevent fornication so they can study harder and we all know that nobody would want smelly junk and it would be a punishment. by then he or she would have a cheese roll or two down there.