After Columbine....a co-worker of mine came up with an online business plan for making such backpacks and briefcases that were bullet resistant. It never worked out...lack of funds (we are cops, what do you expect) but it was a good idea.

Don't forget that a couple of hardback text books and the backpack moved from your back to your chest would do a lot for stopping a 9mm round and will most certainly stop a .22 caliber round. Next time I go to the range I'll try to shoot some old college textbooks I have and see what kind of stopping power they have.