Outright prank. OK, this enters in the TMI region but the ammount of protein in semen would require more than two decades and several “donors” to clog a pipe of standard size (let´s say 5cm wide). Not to mention that most proteins in the biological material are more or less hidrosoluble and it should left to dry for some time.

Yes, but one would imagine that semen sticks in the hair, providing a better clog.

Though I doubt the school administration actually post this, I wouldn’t go so far to call it a prank. More of just a group of people trying to get the point of “hey, other people use these facilities too, you know!” across. Prank implies some level of malicious intent.

Actually, it just might be true. Not semen alone – but with the hair, they have the tendency to stick together so that the semen will harden as it sticks to the lump of hair(s). Yum.

Try masturbating in a shower constantly and don’t remove the hair that gets clogged into the plug. See what happens in a public shower space that is used by several males…