The NRA has come out in favor of armed guards at schools, which while it might help, probably won't work as well as one might hope. Think about it. One armed guard at the front door of every grade school, middle school, and high school. Probably several at the high schools as those are large buildings with multiple doors. And what about colleges? Do we really want the TSA groping students on a daily basis? Because ultimately that's who will get the job. They're a branch of the government, and they're unionized.
Larry Correra has a better suggestion that calls for some of the faculty to get trained, get a CCW, and carry discreetly on the job. This would be far more effective. From the point of view of a potential shooter, a guard at the door is one person who can be ambushed, giving access to the target-rich environment inside. Knowing that multiple persons inside, identities unknown, are armed and will oppose him, the shooter looks for a less risky target, like a movie theater with a "no guns" sign out front.
The problem I see is that in order to get a job inside a school building, one must first survive 4+ years of Ed school, so the mindset may not be there. In the second place it places the teachers union in the position of opposing all guns on the one hand, and demanding "combat pay" for the CCW members, perhaps to cover ammo and fees for one range trip a month on the other.
Of course it's possible that if such a policy were implemented, more teachers than you'd expect might come out of the closet for the training. After all, it would be "for the kids".