2) Arming the other two-thirds would presumably cost at least $2.5 billion. The median police officer makes $55,000 per year. So figure that arming the rest of the nation's public schools would cost at least $2.5 billion. In the context of the vast federal budget, that's not a lot of money. That said, it's also true that school shootings and murders are relatively rare. In 2004, 21 students were murdered on school grounds, while 1,513 were murdered outside of school. So one question is where police resources could best be put to use.
3) Some research suggests that armed guards make students feel less safe — which might hurt school performance. A 2011 study in the journal Youth Society found that the presence of armed guards in schools made many students feel less secure at school. (Though there was a racial divide: White students felt less safe, but there was no change for black students.) Another journal article reported a few detrimental effects from the presence of armed guards and surveillance cameras at public schools.
That's not a trivial consideration: "We know that students who feel safe, supported, and connected in school (psychological safety) tend to have better social, behavioral, and academic outcomes," Eric Rossen of the National Association of School Psychologists told me by e-mail.
4) It's still unclear what effect armed guards would have on school shootings or other types of violence. It seems intuitive that having better security at school would stop shootings, but there isn't a lot of good research on this. One 2009 study found that schools with "resource officers" — security officials, though not necessarily armed — did report less criminal activity. But the authors lamented that there's little evidence on the effectiveness of security measures like surveillance cameras, metal detectors, or armed guards.
And what about mass shootings? It's worth noting that Columbine High School had an armed "community resource officer" on duty the day that two students shot 12 of their peers and one teacher. So armed security guards can't stop every death. On the other hand, it's worth noting that the same can be said about some of the gun-control legislation being discussed right now — the federal assault weapons ban was also in effect during Columbine, after all.