There is no way to absolutely prevent it right now. Training and education will also help to fill the gaps. The issue really isn't the gun anyway it's the person using it. If you figure out how to keep out all the guns. Then there will just be stabbings and once you figure out how to keep out all the knives including the ceramic ones that don't set of the metal detector then you have to worry about the baseball bats, screw drivers and anything else that can be used as a weapon that is already on school ground. Guns make it to easy to hurt or kill a lot of people but there not the problem in the end and focusing effort on eliminating them is just going to shift the problem onto something else.