I had a long talk with a friend of mine who is an Iraqi war veteran. He is trained on all kinds of guns. He told me that in shootouts with police, the police often (more than half the time) don't get the shooter. Plus he told me that it is very hard to train ordinary civilians to shoot back to kill. He was appalled at the thought of arming teachers.
What chance would a first grade teacher have against a gunman, wearing a bullet proof vest, who is firing a rapid-fire gun with endless bullets? And now think about the potential for harm that could come from teachers walking around a school packing pistols.
It seems to me more guns would make the situation worse. I really liked the link that Imagine Wisconsin posted here. Let's try to head off these situations with adequate funding and services to schools.