I see a few problems in this discussion that reflect the real issues we have. I am going to do my best to explain my thoughts without sending the wrong message.
School shooting happen and this is terrible. Whats worse is that it takes a bad thing to happen many times before solutions are found. Here we are with unprotected targets. We need to protect them. So we arm up the last line of defense, the teachers, then work our way backwards to the first line, ( some would say its mental health, other gun control ) but I say its the front door. If schools were built like a base, checked in gates with guards, no shooter shall enter.
There are other options, but this is a loose idea. Stop the guy from entering the building, you stop the need for armed teachers. Prisons, military bases, and embassies are the most secure spots on earth. Why shouldnt we follow that model and add to that schools? Follow the model of the military. A car/bus drop off point, where children go around the corner of a brick wall out of site instantly, an armed guard or two at the entrance, maybe one in a tower, children are then taken by teachers to the main building where they go through scanners, into the building.
Its terrible to think of that, but, its worse to deny the fact that this would be a solution. Of course there are going to be problems with this idea, but the general idea would work. I cant think of all problems and their solutions, unfortunately...