As a thought experiment:
There are approximately 120,000 elementary and high schools in the US.
If we put one armed guard in each school and pay him or her $35,000 a year, absent all of the vast administrative costs associated with such a program, that's more like an outlay of $45,500 for each one of those guards. That's $5,460,000,000 just in compensation and benefits for guards. Yes, some schools already have guards, but we have to allow that some schools could require more than one lonely guard, so the number seems conservative if anything.
After all, we're not even considering colleges and universities, which largely do have their own security staff and which security staff has proved utterly incapable of preventing such attacks on college grounds.
Now, that money is being spent to prevent school shootings. "School-related violence" accounted for 38 deaths in 2010. (Which would work out to about $143,684,210 PER STUDENT life saved, IF we agree that a security guard would prevent EVERY SINGLE ONE of those deaths.)
On the other hand, somewhere between 5-6 children die every day in car accidents in the US. The equivalent of Sandy Hook massacre every 4-5 days all year long. How much could FIVE AND A HALF BILLION dollars do to reduce those deaths? And wouldn't it be amazing -- truly a worthy goal -- if we could bring the number of kids killed in car wrecks down to even 10 times the number killed in school shootings?
But we'll spend the money wherever it is most sensationalistic to do so.