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.
Look at the Detroit Police Station shooting. The cop on the front desk is popped first. The cops were fortunate that they had three more armed and at least one wearing body armor. The shooter had a 20 gauge Mossberg pump action (maximum 5 to 8 shell capacity). You obviously are going to expect more trouble in a police station than a school.
A class change or a school assembly is a particularly scary concept. Could you imagine someone opening up with a Saiga 12 gauge semiautomatic with 1 buckshot and a 20 shell drum? Within seconds sending 320 deadly pellets at a crowd. Lanza had a Saiga with an extended magazine in his trunk.