From Slow Facts:

The solution is obvious, if invisible. The researchers who study school security told us what to do over a decade ago. Murderers stop killing our kids when they face an armed defender. The defender’s response time predicts the body count. The SRO can’t be on the bus before school and on the bus after school, but the bus driver can. The SRO isn’t at the choir practice before school, but the choir director is there. After school, the SRO can’t be at the ball field and in the music room at the same time, but the coaches and band director are certainly there.

Armed defenders are better than the statistics would indicate. The advantage of volunteer school defenders is that they are close to every student all the time. A mass-murderer never knows who is armed and ready to stop him.