What did I just say? You think a person of stature with combat training can't break up a fight in any other manner? If it was your mother and her sister fighting, would you tackle one of them into a concrete floor?
Get real. One of the very first lessons I got in my foray into martial arts was a restraining hold, which was specifically introduced as how to stop an attacker without causing them lasting harm. If I, a beginner, was given such a lesson, someone who sticks to it in a professional context can sure as shit do the same.
Here's another thing that gets me: Thinking that even if something is "absolutely necessary" that everybody should just absorb the consequences. It doesn't really matter that we teach kids they'll face brutal violence to ensure compliance, that's ok because they might kill each other otherwise! Scary!
You don't seem to understand the situation in the more underfunded schools in areas with poverty and crime. There are kids that stab each other with knives, brutally beat people with clubs, and many other different kinds of almost-murder. So yes, in fact they
will kill each other. I'm against using physical violence in lesser circumstances, but by the time you're swinging at someone with a knife, the immediate priority has moved from "teaching kids to respect each other" to "making sure no-one dies." Therapy and other such measures can be administered after the fact, but at the moment the goal is to prevent this person from hurting anyone else. If my sister had a knife and was ready to kill my mother, I would do the exact same thing.
Besides, in these circumstances, the officer has very, very little time, even a tenth of a second, to react. Flinging your body at the attacker is the most instinctive and the fastest response. I doubt the girl would get any serious injuries from that, perhaps a couple stitches in a couple places, but the overall result would be much better if the knife strike was allowed to hit.