Look at how that same argument fails to apply to suicide statistics being reduced by removing access to guns or falls from bridges. You're trying to apply a logical train of thought to an inherently illogical act of homicide.

But even if you can apply that, it doesn't work. The gun and ammo you can fit in a textbook is smaller than what can fit in a backpack. Backpacks are good at carrying things, that's why we - and school shooters - use them. So if someone wants to pack heat, they'll be less effective.

Maybe the effect from this is insignificant. But how sure do you have to be for the risk of dead children to be less bad than someone not being allowed a cute backpack?