I think the cost would be infinitesimal. Weapons are cool, but not essential.
In a way, doing without weapons would be fun. Teach the kids martial arts. Their bare hands would be their weapons. What I like about martial arts is that it's a bit like mathematics. In math you can't fool yourself thinking you know more than you do... your own limitations are right in front of you, in the blank sheet of paper reminding you that you're no Grothendieck. In martial arts you also know your limitations. If you don't, you get your ass kicked really quick. In this age of grade-inflation where all kids are believed to be geniuses until proven otherwise, knowing where you stand relative to your peers is good. Harsh, but it builds character ;-)
I've got nothing against martial arts, but AFAIK, they weren't associated with the observed reduction in criminally violent gun use.
Assuming that the effect is real and significant, why don't you find it important?
I'll state my bias up front - I'm all for reducing criminal violence. (However, that means that I oppose things that don't reduce criminal violence because they make it harder to do experiments and I think that restricting people for no good reason is bad.)
At what cost in effectiveness?
If you don't tell the loons to get stuffed, your life is run by loons.