Disclaimer: I once owned Harry Potter, but then my alarm clock rang. Not really mine, it turns out.
The professors were to blame. Giving teenage girls with a well-hidden love of mischief whole periods of time when they were some of the few students out of class was blatant irresponsibility on the part of the staff. Some trouble was unavoidable, really, they ought to expect worse.
Satisfied with her justifications, Minerva McGonagall gave a small smile of consent to her best friend, Rolanda Hooch, and took out her favourite eagle feather quill. Thank Merlin she hadn't gotten prefect, it had been disappointing at the time, but Minerva would never have let herself do this if she was in a position of responsibility.
This particular mode of trouble making wasn't actually like her, it was a bit too sneaky, she would normally prefer a nice, straightforward prank (Minerva was nothing if not straightforward). She did, however, like a good joke, and was willing to accommodate her friend as long as Rolanda's idea didn't lose them House points. Pulling out the list they had made the night before, they began to copy it name for name onto the wall.
It took less then ten minutes for the two girls to leave their permanent mark on Hogwarts. They would have preferred a Quidditch cup, but being the first students to place a rating of boys on a bathroom wall was something, too.