Not to deflate all the genuinely entertaining disgust and outrage, but exactly how reliable is this bit of news? There's been quite few stories like this circulated by websites of dubious credibility, and most of them have turned out to be about as factual as that story about Bigfoot opening a casino to fund his secret moon base.
Introducing firearms into a classroom setting will completely undermine what is (or at least should be) the purpose of that setting in the first place. A classroom should be an environment of intellectual curiosity, where students should be free to question and discuss ideas. Once you introduce armed teachers, you create an environment of fear. The students will either fear the teacher, or they will come to believe that the teacher fears them. Once you've gone down that route, yes, there might be some instances in which you could reasonably argue that the students are physically "safer," but would this be worth the loss of what the classroom environment should be? No, not in my opinion. It would be tantamount to admitting defeat, because it sends a clear message that we've accepted that our lives are and will be defined by violence and weapons, rather than thought and communication.