There is no such thing as pullet proof material. The vests that the military and police wear are not bullet proof just bullet resistant. If you want something to carry around that is bullet proof then you will need to carry around a shield made of Kevlar or solid steel. Even that can be defeated.
While a bullet-proof backpack sounds like good protection, it is just feeding into the emotional turmoil of fearful parents. The odds of a child having the backpack handy, and then being able to use it against an active shooter are very tiny. It would be better to have more protective measures in schools, like security doors on the classrooms, or armed guards on the grounds. These cowards who shoot at unarmed people will not want to attack a defended or hardened location. Besides, very rarely do the shooters have legal weapons, which means they could also have stronger ammo, which will blow through this 'bullet-proof' material like it was water.
Well that term is wrong for a lot of people to use as you talk to many people and they think bullet proof means just that. It does not stop many bullets. Even the ones it can stop hurts like heck. Otherwise it can not slow down or damper a bullets effects at all. Any bullet fired from a rifle will go right through any bullet resistant vest made.