The local guys who made bullet-proof backbacks have made the national news.

Cnn.com ran the story about the backpacks.

I am sure it was a lot of fun to fire bullets into a backpack and all, but this idea can't go anywhere. First, most schools in Eastern Mass no longer allow students to carry their backpacks from class to class. In the early nineties, administrators and schools committees agreed that back packs were too heavy to be carried all day long, so they banned them from the classroom.
I have worked in three different schools with this policy, both in the inner city and the suburbs. We didn't even allow most students to have cell phones as texting has become a giant distraction. I have never seen a student attempt to cheat with a cell phone, but they do try to text each other during lessons, then they do poorly on tests because they were texting instead of paying attention.
Now, if a normal, light-weight backpack has been declared off limits for being too heavy, I can't imagine the schools saying it's okay to bring in body armor on the off chance a killing might happen. In the schools in the most dangerous parts of the city have metal detectors anyway.
THe only solution is more support in finding and helping counsel the students who would most likely have violent outbursts. Obviously, the government doesn't have the money to insure the kids all stay safe and get the counseling they would need, but it's not the end of the world. Your kid is much more likely to be killed in a house, in a pool, or crossing the street than they are to be killed in a school building.
So, if bulletproof backpacks are out, what can you do to minimize your child's risk to school violence? Glad you asked. You would need to be willing to advocate for other children than your own. If you hear your kid talking about someone getting bullied, even if your child is not involved at all, call the principal and ask them about it. This does two things. First, I'm sure the principal has no idea what is happening with this specific instance, so it puts it on the radar and he can bring in the students involved and the teachers who can help and coordinate and effort to make things better. Second, it lets the principal know that you are a concerned and involved parent, who is willing to call with a problem. This means the school knows that you are going to call them when the grades come out, so it's better the teachers make sure your kid learns what they are supposed to before the test, and they'd rather talk to before the grades come out than after.