According to the National Center for Education Statistics, there are just under 100k public schools in the USA.
Each school should have a minimum of three entry points, and of course given the push to regionalize into mega schools, there are likely many more.
So let's figure three armed guards per school. Theyll have to be paid similar to a police officer, so, let's just say $75k/year each. Now, the actual burdened cost of labor is more like $150k/year each. So that means nearly a HALF MILLION DOLLARS per school to have each one guarded. That's $50 BILLION in labor per year just to have guards in each school.
And schools drive property taxes which most dont want to pay...
So why anyone would push for guards in schools, when actuarially speaking the chances of children killed or hurt is infinitessimally small, is beyond me.
Arming of the "contingent of the willing" of staff to me is a more reasonable thing. But that will also, undoubtedly, create a bureaucratic training reqirement, some element of liability for injury and damages, and a LOT of other red tape as well.
So in other words, there is no clean and clear answer here, and the fact that you have a high density of children, makes it an even more touchy subject. I'd hate to think that we have stooped so low as a society to have to take precautions akin to El Salvador, but perhaps it is the case. Wonder what the statistics are for school shootings in the UK (which is more violent overall than the USA), as well as Canada, Germany and Switzerland. Is this an issue at all?