@Ryan - Of course we don't want to arm teachers and other private citizens in the hope that it will reduce or end these kinds of shootings. I shudder to think how untrained (or poorly trained) armed people would react in such a situation. Take for example the "Batman" movie theater shooting. If everybody in the theater would have been armed, I suspect that many more people would have died due to the panicked movie goers wildly shooting in an attempt to defend themselves.
IMO, one of the biggest, if not the biggest reasons we have more gun violence than other first and second world countries is that we have a "gun culture". It's culturally drummed into our heads from an early age that using guns are a good way to solve problems. Our movies reflect it. Our computer games reflect it. Our literature reflect it. Our foreign policy often reflects it. Unfortunately the reality of using guns to solve problems rarely matches our cultural beliefs. Until we can change this gun culture, we will continue have gun violence incidents.