Some folks said "Well there was a cop, trained, and the shooting still happened". Maybe so. But did the shooter even know this? In any form of security, be it a bouncer at a bar or corporate data in a locked office, the mere presence of security is often times enough to curb an assailant.
Schools are still perceived as killing fields. Fish in a barrel. We have to erase that notion and let any wackos know that yes, the schools are protected, and there will be apprehended. You won't be able to just kill 15 people and escape (by taking your own life).
It seems to be THE reason you don't see shootings at malls, the airport, etc. Killers know they will become caught. They know there's security ready to do something. They need to think twice about going into a school with a gun.