Speaking of that, a lot of people misunderstand the criticism against violence in games and movies. There was never an argument that perfectly normal kids would be warped by violence and go out wanting to slaughter people; so Moore didn't really address the issue seriously in his movie. The argument was that when violence was common, people would become "desensitized" to it and no longer see it as abnormal, so you could no longer tell when someone had a problem. There are no warning signs anymore if many kids are into it. Parents don't even know everything they're kids are watching or seeing, so they might not even be able to tell if there were a warning sign.
So -- so what if he was locked in his bedroom listening to death metal?
Yes, the point of the Judas Priest reference was to show how ridiculous it was to assume that dangerous behaviour in a teenager is easy to spot. They play violent games. They listen to music where some rapper (who probably went to private school and has almost certainly never popped a cap in anyone's *) talks about gunning down policeman. Ninety-nine per cent of these kids turn out fine, and occasionally some do not. I just don't see how teachers can be expected to spot them (and remember that many of them are spotted) when the parents don't either.
In this country, we have frequent examples of teacher's putting themselves in harm's way to protect children, so I probably have a different perspective.