With all the talk lately of guns in schools (some of it due to the OR teacher fighting for what should be assumed rights), there's a lot of talk out there about why they aren't a good idea:
Students will find them. If a teacher keeps his/her gun on his/her person, s/he shouldn't have to worry about a student finding it.
What about crossfire? A teacher firing back increases the odds of hitting an unintended target, perhaps, but only if the teacher doesn't wait for a clean shot. Also, if someone's dead set on killing folks, s/he needs to be taken down. More lives are risked by waiting than are risked by firing back.
What if the student takes it from the defending teacher? The odds of a student being able to take away a gun are pretty low, especially if the teacher has chosen to deploy the weapon.
Teachers aren't supposed to fight, but to teach. Well, there's certainly a little truth to this one, but we don't live in a utopia. You can't teach the dead, so you'd better protect the living.
Feel free to disagree with me, but I think that incidents could be stopped quickly if a few armed people were available, rather than having a "weapon-free" environment.