He didn't even bring the gun into school. It stayed in his car. Your own loving words what would a metal detector do in this situation?
It sure as stuff wouldn't keep him from shooting up the school if that was his intention.
I mean, it could be considered conspiracy to commit murder, also it depends if the car was on school campus or just on the road.
Though, what the hell kahnu, you're seriously complaining about how your school doesn't have metal detectors all over the place. Huh? That many security guards is a loving luxury, either the school is rich or very paranoid.