Some unknown wag had carved the words 'The Dick Seat' onto the back of one of the chairs in our French classroom. As if controlled by some higher force, the location of the dick seat could never be reliably predicted from one lesson to the next. It was, of course, accepted without question by everyone that sitting in the dick seat would make you a dick. In some kind of ghastly parody of Musical Chairs, you therefore had to get into the lesson as early as possible to ensure that you secured a normal chair.
The seriousness with which this was treated was such that even the entrance of a teacher wasn't enough to put a stop to the titanic struggle between two boys having a tug-of-war over the last remaining safe seat at the start of a lesson.
I still check the back of every seat I sit in.

There was hardly a single chair at my school that didn't have a penis drawn on the front of the seat. You would often find that you had suddenly acquired a new marker pen cock, which was poking out from between your legs in a suggestive manner.
The practice became so widespread that the headmaster had to address the issue at assembly. He prefaced his announcement with, 'This is not in any way a laughing matter,' before continuing, 'It has come to my attention that male genitalia have been drawn...'
Despite the headmaster's protestation that the word 'genitalia' was 'not funny', the speech was swiftly abandoned.