What the NRA gets right -- and wrong
Israel does this. School security is a serious concern in Israel, given the constant threat of Islamic fanatics who would like nothing better than to carry out a mass slaughter of Jewish children, and who are often better armed, trained, and motivated than the mental cases who periodically attack schools in the US. Armed guards seem to work.
They also seem to work where we in the US use them, at places we're actually serious about protecting, such as banks and some government buildings. They don't create an armed-camp atmosphere or an increased level of danger. And notice that almost all mass shootings in the US happen in places that are designated gun-free zones, not in places that do have armed guards. Even mental cases know they can kill far more people in places where there will be no effective resistance until the police can arrive from some distance away.
Opponents of armed self-defense in Newtown-like scenarios seem to imagine that it would result in some sort of random crossfire which would kill even more people. This would certainly not be the case with properly-trained guards. Adam Lanza killed 26 people at the school, shooting most of them multiple times. There is no plausible scenario in which a trained guard shooting Lanza as soon as he started his attack would not have resulted in a much lower death toll. Even a poorly-trained civilian shooting back at Lanza -- yes, even if that civilian accidentally hit one or two innocent people -- would probably have resulted in a much lower net number of deaths. Perhaps more to the point, Lanza would have been unlikely to attempt his attack in a venue where he knew he himself would be shot as soon as he began.
There is, of course, another side to security in Israel. From the link above:
In Israel, no one carries a weapon without the Israel Ministry of Defense knowing about it. The person carrying the gun or assault weapon has gone through a security background check, trained in the use of gun safety and is registered with either the Israel Defense Forces or the police that he bears a weapon.
I've always been dubious about gun control. It was easy to get alcohol during prohibition and it is easy to get marijuana now, and that's in the face of a total ban, not merely laws to restrict access. No gun control law would have kept Lanza from getting hold of the weapons he used; they were owned by his mother whom he also murdered. We probably should implement stricter background checks and broaden the categories of people who cannot legally carry guns, but we shouldn't expect this to have much actual effect. In the US, a person who is determined to get a gun and doesn't mind breaking the law will always be able to get one.
Finally, the NRA's LaPierre is not only wrong but despicable in blaming violent movies and music videos for the problem. Tens of millions of people enjoy such entertainment without becoming violent. If violence portrayed in entertainment led to violence in society, Japan would be the most crime-saturated country in the world; it's actually close to being the most crime-free. Hunter-gatherer societies suffer staggering rates of murder and other violence without violent entertainment (or guns).
There are other countries where gun ownership is widespread, yet gun violence is rare. The reasons seem to be a matter of culture rather than law. Changing culture is, unfortunately, much more difficult than changing laws, but a more compassionate and supportive culture would have benefits far beyond a reduction in violence. We really ought to give it a try.