But yes, I agree that guns are only a tool, and that the underlying cause is psychological and/or cultural. No matter what we do with guns, it won't prevent future incidences; I just don't see how people can view a situation like this and think "oh, if the teachers had guns, everything would be all right."
As far as arming teachers, it's not a perfect solution, nor is it going to "make everything alright". When you have people like this who end up being so bent on destruction and violence things are not going to be alright, it's a matter of degrees. However, there have been several incidents of teachers and civilians putting a stop to mass shootings, likely saving many lives. These often don't get as much news coverage precisely because the body count is lower and thus the events are less newsworthy. For example: http://en.wikipedia.org/wiki/Pearl_High_School_shooting