I think if a school is located in a high crime area, these kinds of measures might make sense, not so much to prevent mass shootings, but to prevent violent crime in general. It's unfortunate to have to create that kind of environment at a school, but there are limited cases where I can see where it would make sense. However, most schools are not located in high crime areas and don't typically have many problems with shootings. Requiring all schools to have metal detectors and armed guards in an effort to stop a random mass shooting is going to create a feeling that the school is like a war zone. That's not a good environment.