Putting guns in the hands of teachers will lead to at least some shootings (accidents or otherwise) that wouldn't have happened otherwise, and simply move the mass killers to another patch of turf.
Why? You're making assumptions here that are very probably not true. Accidents? These teachers will be trained. Do you see security guards outside the Whitehouse accidentally shooting tourists? Fuck's sake. Deliberate? If someone is that way inclined, then it was probably going to happen anyway, and, as a teacher, one imagines they would legally be able to obtain a gun. So you're not stopping any accidental or deliberate shootings by not giving teachers guns, that's absolute bollocks that hasn't been remotely thought through.