How about just removing the "Gun Free Zone" designation and allowing those teachers who own guns to conceal carry? Not mandated, just allowed.
The gunman then must consider there may be resistance to his plans. Lanza shot himself upon hearing sirens. It seems to me that he knew he would not come up against opposition for a period of time, and when he did - he ended it.
The following link shows state by state statistics and information gleaned from interviews with convicted criminals. In short, gun ownership works.