Its not necessary to harden the schools enough to defeat every threat only enough to place doubt in the mind of perspective shooter as to weather a school is soft enough to allow his rampage to be successful. Even if a shooter decides to proceed with his plan, he will have to think about a possible armed opponent in every room. That certainly will complicate the shooters planning. Anything that slows the attack, gives responders more time to arrive and intercede.
If armed teachers or other plains clothes security assets are permitted its important that they are not identified as such, not only because it might make them first targets, but because not identifying those carrying, means that would be shooters would have to assume that there might be many more than there are. Create enough doubt and many perspective perps will choose other targets, or just stay home.
