So lets look at the stats:
There are 98,917 public schools and 33,366 Private schools (2009-10 figures)
At a mean $26.74 per hour (2010 national mean), say a 40 hour week to cover arrival and departure times as well as teaching time that comes to $141.5
Typically with 36 weeks per year that is $5.094
Now there are only 644,300 Police Officers in the whole of the United States, therefore during the school year 132,283 of these or approximately 1 in 5 will be spending their entire working week on school protection detail. So there would have to be some sort of scheme to overcome the short fall in crime prevention, investigation and protection of the general public.
Now surely we can find cheaper ways to protect the school children from the bad men with guns, rather than having good guys with guns (another NRA quote) there?
Update sorry I was miscounting my zeroes Billions were millions and trillions merely billions