Ballistic armor companies are marketing protective products designed for the military to parents and schools. Some people see the items as unsettling but prudent; others find them infuriating.
How likely are you to be wearing the backpack and get shot from behind?
Not trying to say it's not a racket but the backpack could easily be used to provide cover in all sorts of situations. It's not like it can only be handled on your back, you can flip it around, hold it over your head, get down in the fetal position and hope it covers you enough to keep little johnny from shredding you with daddy's ar. Pretty versatile actually.
I guess it's really down the the mindset of the attacker. Going for a high body count for internet clout would cause that. Beef with a specific class for percieved wrongs, probably not.