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?
There are an unlimited number of things that risk dead children. Chewing gum, shoelaces, bicycles, model rocketry, shop class, video game marathons, crosswalks...
How sure do we have to be to ban them all?
There is in my opinion a very clear danger associated with setting up draconian surveillance states borne out by history. How sure do we have to be before we ban those?
Suicide usually involves sevee depression and such a lack of motivation that opening every blister packs to overdose is a deterrant. It is very different from a desire to spree kill.
Besides if we deal with this low probability extremes shouldn't we factor in early blooming girls who kill themselves after being bullied and shamed over carrying their feminine products without any privacy?
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?