Then they will find some other means to do their thing. School buses? Malls? Day cares? The list is endless. Not to mention school shootings are far from the only form of mass murder - movie theaters recently too.
Unless you are ready to sign up for a fortified, bunkered America in all facets, "security" is not a solution to this problem. The vulnerable surface area is all public spaces.
I'd be much, much more concerned about the state of mental health care in this country. No one in their right mind goes and shoots children, nor a theater full of movie-goers - these aren't crimes of opportunity, they are indicative of severe mental disturbance.
Inevitably when tragedies like this come up we get into a big fight about gun control and then forget it ever happened. So rarely do we see any real discussion about preventing someone from seeking out the gun in the first place.