Arming teachers is a bad idea. Given the power trip some teachers go on, I can very easily see it being used to stifle dissent and debate, even or perhaps especially in classes where such things are essential.
Given the number of innocent bystanders that are guaranteed to be present at a school shooting, I find it much more likely that arming teachers would only lead to more injuries and deaths. One little-reported fact is that there was an armed bystander at the shooting of Gabrielle Giffords; they almost shot the person who ended up holding the gunman's weapon after he was disarmed.
