Again, I think your standpoint is one of assuming most people are as particular as you when it comes to weapons. And it's probably true that most people are, it's the few that aren't that I think pose the problem. And it's somehow letting these few slip into a school that concerns me.
This pdf is long, but it lists the ccw holders that killed people with a gun.
http://www.vpc.org/fact_sht/ccwprivatecitizens.pdf
Granted, it was usually spouse, friend, or self, but the person who fired on the crowd in Arizona, and shot Gabriella Giffords, for example, was legally carrying a concealed weapon:
The guy that shot 6 people at a Seattle cafe was legally carrying a concealed handgun. Sadly, his family knew about his mental illness and tried to get his permit revoked,
I know these are not the norm, but they are out there.