As a parent and grandparent and many years longtime school classroom assistant volunteer, I can tell you absolutely that very few professional educators wish to carry a firearm in the school or even have it in the classroom.
Although it's true that "the general populace has trended towards more and more armed in the last 5 decades", yet teachers do not want to be armed in the classroom.
For those who advocate for guns in school, I'm sure you believe it to be a significant deterrent to the relatively rare occurence of mass shooting. However, I'm more certain that guns in school would be a much more common and significantly more adverse deterrent to a healthy learning environment.