He doesn't answer when she asks him why it was in his cupboard. But right after she asks, the two bad guy students walk into the room. One has a knife to Casper's throat. The other one says "it was our bathroom". It's clear Casper isn't involved because they strap him to a table to cut him.

how was it never explained? If you paid any attention you'd have seen that during one of the final scenes in the prosectorium two villains, one blond (hein -holding a knife to the caspar's throat) and other a long-haired stoner said that the cupboard was theirs and that they both were a part of AAA!.