The problem is the easy access of guns, the moment you force potential would be school shotters to go to a gang member/ drug dealer type to get a gun not only does they risk getting caugth, you also give them time to rethink what they are doing, since I doubt it would be easy and fast for a kid to do this, they might get scared or have the time to calm down and think rationally.

In a fit of rage people might grab for the nearest weapon, and they are more dangerous if it is a gun than say if it is a knife or a broken bottle of booze.

Of course if you still think gun ownership is everyones right then you should probably start to rethink the way you make them avalible, as well as how you portray gun usage in fiction and elsewhere, I can hardly watch an American action movie without worrying about the numbers of "Bad guys" shot down in cold blood by so called "Good guys" such vigilante behavior makes it more acceptable to kids to use guns if they think themself the "Good guys" and all their teachers and co-students as the "Bad guys"