What happens to expose Jack's mental instability in the novel, Just Another Hero?

Called Crazy Jack because of his outlandish clothing and strange behavior, Jack Krasinski plays snare drum and cymbals in the high school band, but also is known for carrying his instruments around and playing them at odd times during the day. When he had a psychotic break and takes his fellow students hostage with an assault rifle, it is revealed that Jack's love of loud noise is really an effort to drown out the voices he hears in his head.