I don't disagree, any shooting at a school is major.I think 20 dead, 15 dead, 10 dead, or 4 dead are ALL major.
But the kind of protection that we are theorizing about here likely would not have made a difference in about 80% of that list. Those are usually cases of kids sneaking a handgun into the school and having a specific agenda (ie. specific target) or possibly a heat-of-the-moment altercation.
We are talking about someone walking through the door with an assault rifle to kill as many people as possible before they die.
In the grand scheme of things, there isn't much difference. Both are a tragedy. But the efforts that we are talking about to limit a nutjob from getting into the school while class is in session only applies to a fairly specific situation, and there have really only been about 4 of those.