He tries to disarm the bomb, but the principal comes in and arrests him, and then the one bad guy is randomly running while some random, ominous figure is talking to him.
Well, the principal is a dumbass, in case you missed that with the action figure. He completely forgets that Kris warned him about the bomb, so as far the teachers know, he's there trying to arm it rather than disarm it. I mean, imagine if you walked in on a kid wearing a leather outfit and a bomb in his hands.
Secondly, the bad guy was the one who actually armed the bomb, and the ominous figure is telling him that he's lucky Kris went to jail (being him the only one who could identify the bad guy) and no one else saw him. It's all part of the plot. I promise it'll make somewhat sense later.