I strongly believe many of the most outrageous instances of this type of event stem from a failed mental health policy of the last 40 years combined with massive failings in the enforcement of mental health at risk individuals.
I agree the risk for this is high. I am not suggesting that the NRA's endorsement of armed guards is in any way wrong. Rather that by the very nature of the good guy vs the bad - the armed good guy will always be reactionary. Meaning the bad guy attempts a murder or other serious offense in the first place.
Somehow tying this into video games which does nothing to impact that "pre-attack" problem at its roots. That does not seem too beneficial to America long term or even gun rights. What happens when a nut job passes the number of victims as the Bath killings in the 20's (albeit w dynamite)
One of the difficulties I suspect with armed guards in schools is perception. We know for a fact that certain private schools have armed guards. The fact that they are, technically, private gives them a degree of flexibility that many public schools lack. What many people may not realize, however, is that some public schools (particularly high schools) already have armed police on campus. While I don't think high schools have as many "narc's" as they did when I was a kid, I suspect that some still have them.
Trying to place plain clothes officers on campuses, I suspect, becomes something of a logistical challenge. Many public schools have more than one entrance. And many are campuses containing multiple buildings. While we know that the officers role would be both preventative and reactive, where do you strategically place the officer during the day? People would want the officer to confront the attacker before anything bad happens. Well unless we want schools to feel more like concentration camps, we can't exactly place the officer by the entrance where they are visible to the public (and expect them to deter attackers, reduce crime, and police the area for someone trying to gain access through other means). In reality I suspect it pretty likely that an on-site officer will not be terribly pro-active in stopping whack jobs. They will more likely be reacting to issues when they happen.
Since people seem to not be terribly receptive to the idea of arming teachers, maybe a more cost effective alternative would be to install panic buttons in classrooms. The challenge with that would likely be two-fold. One, how do you keep students from pushing the panic button for kicks... and where do you place the panic buttons? In my last job I supported a technology upgrade at the Flagstaff Unified School district. One thing the I.T. guys up there told me was that the teachers moved their desks around in the classrooms on a regular basis. So to place a panic button on the teachers desk might require wireless buttons... or an easily accessible location in the classroom that some kid could not easily get to. Or maybe all of the teachers get those "I've fallen and can't get up" wireless pendants.