Gun laws don't have anything to do with the guy going out and starting to shoot. If all students were required to open carry, the joker still might have started his dastardly deed.

If the students were required to open carry at all times, the guy might have felt the need for more and more powerful guns. This could have slowed him down to the point that he might have abandoned his plan.

However, if things had played out basically the same way, the hero who charged the gunman and received 7 gunshot wounds as his reward, wouldn't have done that. He would have simply shot the gunman and the problem would have gone away.

Gun laws don't make the problem. What they do is make the problem worse.