Students should not be wearing bulletproof backpacks. They already have 5 to 6 subjects worth of stuff in their heavy bags already, and making backpacks bulletproof would just make them an even more inconvenience. The shouldn't have to wear anything bulletproof to be honest with you. Jut keep the schools safe and this will be avoided.
If you are going to go to that extreme bullet proof vests would be more effective. But this is kind of like preventing rape by wearing chastity belts. Mass murderers can just resort to bombs if they know all the kids will have bullet proof backpacks. It doesn't get to the root of the problem. We have to address the very real problems that make people resort to violence, and that is not such an easy fix as buying a backpack.
Incidents like the tragedy in Newtown often have the effect of causing people to overreact and become paranoid. There is no better example of this than parents purchasing 'bulletproof backpacks'. Statistically, the chance of a child being a victim of a mass shooting is negligible. Furthermore, it is unclear how effective a bulletproof backpack really would be in such a situation.
Bullet proof backpacks do nothing to solve the problem. If you ask me, it will make the situation worse. Kids will play dumb and try shooting the backpacks as jokes. Besides, the backpacks can not always be worn, and they do nothing to protect the front of the children. The issue is the guns, that is what needs to be resolved.