Alegrias that is actually what I said, but to just deal with the gun end of it, doesn't stop it happening again with a different weapon. A knapsack bomb in the same situation would probably kill more children and teachers than a gun can.
The 'nutter' who committed this outrage was out to kill, and presumably, children and teachers made an easy target. if he had not used a gun, he may well have used another weapon. It could just as easily have been a home made bomb.
The first and most important thing to do is find out how the 'nutter' came to be free to go to the school and to do what he did and make sure that no-one else like him is allowed to wander free and commit another such rampage.
Then you look at the availability of the chosen murder weapon and may well pass measures to make them less easily available, but 'nutters' ind suitable 'weapons' if they want some. I have yet to hear of a gun looking for someone to use it to kill someone.