You will never eliminate guns from those who want them. Criminals/mentally unstable will always find one. Gun laws only prevent honest citizens from having them.
Make a school, or other venue, a 'gun free zone', and you've just given the advantage to the criminal intent on destruction. They know there will be little-to-no resistance.
Arm teachers? Not a fan of that. Nearly all of the teachers I had in school were simply not capable of handling a gun, let alone shoot someone. Can't imagine any of them doing the right thing in the right way when confronted.
Trained and armed security? Better, but still not a great environment for kids.
More proactive scanning for people that announce their intentions? If facebook/twitter can ban someone within minutes of a post, they have the capability.