The most obvious problem with armed school guards
I am not nor will I ever be a student of mass shootings in schools, I do not watch the ceaseless coverage of these horrible events on the television, I do not follow them in the papers. But like everyone else I get the basics of the stories, and they are usually very much the same. A young person schemes to obtain guns, they fantasize about shooting their fellow students, they plan their rampages and then they go carry them out. Generally speaking they themselves are killed in the carnage that follows.
The point is that the killings are planned.
If you put an armed guard in a school and one of the students is hatching a plan to shoot the place up and kill as many people as possible just who do you think is going to be the very first person to die? A bell will ring followed shortly by the noise and confusion of the change of classes and during those moments the guard will be flung to the floor by the impact of of the first bullet. This is a mad-person's equivalent of "Shock and Awe" which you may recall as this nation's preliminary strike at the defenses of Iraq, disabling them before we got started on the real carnage. Its so simple any schoolchild can see how it works.