The Planet of Junior Brown, Paperback

Junior Brown, an overprotected three-hundred pound musical prodigy who's prone to having fantasies, and Buddy Clark, a loner who lives by his wits because he has no family whatsoever, have been on the hook from their eighth-grade classroom all semester.
Most of the time they have been in the school building -- in a secret cellar room behind a false wall, where Mr. Pool, the janitor, has made a model of the solar system. They have been pressing their luck for months...and then they are caught. As society -- in the form of a zealous assistant principal -- closes in on them, Junior's fantasies become more desperate, and Buddy draws on all his resources to ensure his friend's well-being.