I am sure you mean well, but bulletproof glass in the school is not the answer IMO.
For one thing a hostage taker who is already in the school would be protected, It would be a fort for him.For another the expense of bullet proof glass in every school window would be enormous.
A person set on getting into a school would just come in with a busload of kids when the door is open, or he would wait until someone came out or was going in and get in then.
I agree with you. I am not saying bullet proof glass is the answer. But based on this crime it was.
A lunatic can wait until a bus passes and just shoot up the bus. There are a thousand things someone who wants to commit mass murder can do. There is no single answer.