You do realize there are wall phones in every classroom, and each teacher can carry their own cell phone, do you not? You're talking about dozens of people who have the ability to call 911. Past a certain point all you're doing is saturating the 911 line and preventing ambulances and fire trucks from getting to other unrelated emergencies.

Having students alert 911 in that situation is insane. Their priority should be either running away or hiding, neither of which is helped by getting on a phone call. If that's the point hire someone whose job is 100% just doing that and make everyone safer for it.

Ah yes, let's flood the 911 lines with panicked kids and then their parents too. Making sure no other emergencies can call 911. Isn't it great every single other type of emergency a person can have will magically be put on hold until the school situation is cleared up...

Oh, and there's going to be some kids who will think they need to give regular updates, so they'd likely put themselves in danger in order to "just help"