A giant piece of paper on the wall whose sole purpose is to house graffiti. The comments, drawings, thoughts, quotes, song lyrics, and nonsense written on it is generally written in marker. When it gets so almost every inch of the paper is filled with something, it is time for a clean start with a new piece of paper.
The graffiti board we had in our 6th grade classroom was a conglomeration of really neat stuff and utter shit, such as "Lisa and Sidney and Stephanie are cool," which was scrawled over the board (by lisa, Stephanie, or Sidney) at least half a dozen times per page, so the total is probably a very high number. There was often a "not" inserted into the sentence by someone who got tired of reading that same shit every day, and this caused much uproar.