Dave - You missed a very important group.

Kids themselves.

Troubled kids are often very skilled at decieving adults.

It is their peers (fellow students) that are the most accurate judge of character. They are the ones who know when someone is not quite 'right'. We need to encourage our children to talk to someone they trust if they are aware that one of their peers express thoughts of violence, either to themselves or to others.

I'm pretty sure that this guy was 'under the radar'. If anything, he was probably depressed and suicidal.

Suicidal tendencies in youth are often hard to detect.