You missed the amounts of kids that committed massacres, which has made US schools understandably paranoid.

We're a quarter way through the century and you can about count the number that did a good enough job to make national news on one hand. It isn't something that a first pass filter ought to be looking for.

What is the false positive rate of this system?

If you want to solve this problem solve it at the root, not by overreacting to teenagers.