Bad things happen, what you need to ask yourself is what is the best way to be prepared for it. Using Dorner as a "bad example" of what could happen with cops in schools is assinine. Its like saying Lanza shows all gamers on psychotropic meds are child killers, so lets not have anymore gamers or psych meds.
The problem is, and always will be, the criminal intent on mischief and mayhem. If armed guards at schools are so bad, why does the President and other well to do parents send their kids to a school with armed guards everyday, safe in the knowledge a Newtown incident is a lot less likely than at JFK Elementary in the less affluent end of town.