I attended a school where you had to go through metal detectors. It's no exaggration to say they were ineffiient. My children now go to a school where I have to wait in a room that is locked from me being able to proceed into the school, when I pick them up. So I feel more secure? No, not really.
Many reasons have been stated here, but the one I believe makes the difference is consequences. There are none. Anyone can do practically anything they want, with very little or no consequences. Let's quit trying to make new laws, and make jail a place no one wants to return to. The laws are clear, the consequences have been deleted. Criminals give up there rights when they commit the crimes to their victims. Capitol punishment needs to be swift and available for public viewing (if you don't want to see it, don't watch it). Bring back some humility to the person who decides to commit crimes. Don't hide them, and their crime, make it known.