That’s the result of what needs to be done, not the genesis.
The first and foremost thing is to teach the teachers to fight back. Use anything at hand. Make the cost of attacking your charges greater than the attacker expects. Throw things, stab with scissors, use fire extinguishers to cloud their vision then throw it at them. Beat them with chairs, books, rulers, anything.
I’d take a bullet for my children, lord knows I’d take a few for kids I don’t even know (if you recall, back in the 90’s I made the choice to take a bullet for every last one of you). But I won’t take it standing in front of them. I am not bullet proof and make a terrible shield. If I’m going down, I’m going to do my damnedest to take the attacker(s) down with me, or at least buy others time to get in on the attack as well.
However, it is important to remember that teachers are not soldiers, nor should they be trained to be. They’re not firemen, EMTs, or nurses either, but we give them just enough training in those areas to fill in the gaps before the firemen, EMTs, or nurses can get there. Another very important fact is that even though the creation of Gun Free School Zones is mostly responsible for these things happening and that we’re seeing one of these every two years, it is still a rare event. The horror of it, added with the 24 hour news cycle, makes it seem a lot more prominent. I’m not discounting the horror, just the illusion of this happening all the time. There are 144 schools in my district alone (granted, we’re usually in the top 10 largest districts in the country), the chances of needing this are near zero but then again, we train teachers like crazy on fire drills and IIRC we’ve not lost a child to a school fire in half a century.
When we start instilling the mindset for teachers to refuse to be a victim, then arming them with the appropriate tools will follow as a natural progression. However, without the will to fight back, no tool is useful in defense.