I couldn't help but wiki it and found this.
'At an unknown point in time the Hobo was a member of the Bullworth faculty. He occasionally says he should go back to teaching, and calls Dr. Crabblesnitch a spineless weasel. The nerds mention that he was a former teacher in smalltalk. Miss Danvers occasionally announces over the PA system that "contrary to rumor, the old drunk by the school bus is not a former teacher at Bullworth". The exact nature of the Hobo's relationship to the school and its staff is unknown, however.
The Hobo's last name is Grant. One of the townswomen has a line of dialogue that states about a Mr. Grant and the Korean War, and in fact the game's data files list his name as Mr. Grant. It is vaguely hinted by putting various lines of dialogue together that Mr. Hattrick was responsible for the friendly fire incident in Korea.'