What do Sandy Hook, Columbine, and Virginia Tech. have in common? There was nobody there to stop it.
If we could plant someone into the faculty of every public school (and any private schools that consent to it) that was specifically trained to identify and neutralize any and all threats with or without lethal force, there will be someone there to stop it that CANNOT BE TARGETED. If you put someone in a uniform he/she will be the first to be shot if someone comes in with a gun. However, if the faculty member is one of the people "behind the scenes" like a janitor or a cafeteria worker, he/she will very likely be overlooked and will be able to be anywhere at any time.
This tactic is already used by Federal Air Marshals to protect our airplanes. Why not use it to protect our children?