Gentlemen by Michael Northrop

Four friends - Micheal, Tommy, Mixer, and Bones. No one seems to care much for them, so they all look out for each other. But then Tommy disappears. He's run off before but not for this long. Meanwhile, the boys' English teacher, Mr. Haberman, is acting very strange. First off there's the matter of the big plastic barrel he had in class. He won't tell what was in it, but it sure seemed like a body. And he's dropping hints in class that he knows what happened to Tommy. The boys are beginning to find it plausible that Haberman murdered Tommy.

But who would believe them and how do they prove it?