Even so, arming resource officers and teachers must be viewed as a last line of defense. We must stop killers outside the school, as in Israel. Here’s what we should do:

►Districts should post trained and capable armed guards outside as well as inside schools.

►They should also consider reducing the number of entrances to the building, so these guards can monitor traffic.

►Where feasible, schools could install metal detectors to ensure that nobody can sneak a weapon onto campus.