Surprisingly, no one has done this yet, so I did some Googling.  Using statistics available from five years ago, there are 98,000 public elementary schools in the United States.  There are an additional 26,400 public secondary (high) schools also.  This does not include private schools or charter schools (however, one might want to classify those).  For now, I will leave private schools out.

If you were to have at least two armed guards per school (some of these schools are quite large), and you account for population growth (these stats are five years old), you would get a new overnight federal employee head count higher than 250,000.  That's five modest-sized cities.

If you pay them around $36,000 per year in salary, that yields a new annual cost of $9 billion.  If you add in medical, dental, and pension, plus all the paid holidays, sick leave, and other miscellaneous time off, that raises your cost another $4 billion each year for a grand total of $13 billion per year.

There have been 138 shooting deaths since Sandy Hook in 2012, or 11.5 per year.  Those shot but not killed were around 438, or 73.  Depending on which math one chooses to use, if the new guard salaries remain constant (they won't) the ratio to procure school security per student death is $1.13 billion per child lost.  The cost ratio is reduced to $178 million per kid shot, since there are more of those.

I should mention that the reporting on "school shooting" incidents is murky.  Some of these are not mass school shootings.  Some are accidental discharges by armed security already or police responding to a call.  Some are from arguments in parking lots between grown men.  Who knows how to factor that in, but suffice it to say that it is much more expensive in real bona fide cases of school shootings if one factors in these anomalies.  Perhaps the cost would then be considerably higher if we are to use the rhetoric of CNN in its truest applied sense.  And then there are the inevitable raises due to the racketeering of public unions and armed guards wrangling for ever higher pay, and boy would they get it!  How dare you argue for less money to protect our kids?  We'll call this the Pandora's box effect combined with hysterically maudlin politics.

Who knows?  By the time we're done with all this societal psycho-drama, we may be in the statistical average of spending $2 billion per accidental parking lot scuffle between parents on back-to-school night.  Not to say that "protecting our kids" is unimportant, but perhaps it would be much cheaper as well as better scholastically to end all public school entirely and recoup most property taxes.