According to the US Department of Education there were 98,271 public schools in 2014, with some schools being comprised of a campus of multiple buildings. To put armed security in all of those school buildings, in numbers adequate to effectively guard every potential way for a gunman to enter and to insure a rapid response to an active shooter emergency could take several guards per building. We could be talking about a need for half a million or more armed security guards just for public elementary and high schools in our country.
Every guard would have to be trained to excessively high standards so that they would never make an active shooter emergency worse by shooting inaccurately, at the wrong people, or use deadly force by mistake in a situation where it wasn’t justified, thereby killing the very children they are meant to protect. I’m not optimistic that such high standards can be realistically and universally achieved.
Each guard will need to be armed and equipped. Each will need to be paid a fair salary. The number of guards roughly outlined above is close to the total number of police officers already serving in the US right now. So, putting armed security into our public schools is a bit like doubling the size of the police force in the US. Which could cost something on the order of $25 billion per year in salaries alone, plus another few billion per year in training and equipment costs.