Guns. And I'm using that example because well...you know, wacko has a gun, shoots innocent people. It's kinda the same story for each mass gun crime (wacko+gun=innocent people die). Giving more people access to guns, or teaching them that guns are cool as it seems like this "attraction" is trying to do, especially someone as young as 13, is never a good idea.
Edit: And before you start with the "Oh well if one of the teachers had a gun, they would have shot him dead before he killed anyone!" or some other variant that includes another person having a gun and taking control of the situation, don't bother. That narrative doesn't work.