A few things we're all forgetting are that:
1) high school parking lots are generally "secure" these days, or at least gated and fenced, so even being aware that something like was going on if you were walking down the street would be nearly impossible,
2) having a firearm on high school property is a FEDERAL offense, even in defense against a "deadly force" attack, and
3) in many states you're going to get sued in an avalanche of civil suits (after the criminal case is decided) by every kid's parents who's little darling was traumatized by the discharge, use, or even mere sight of your "deadly handgun".
If something like this were happening in the parking lot of a WalMart, or on my street, or in a city park while I was legally carrying, I wouldn't hesitate to intervene somehow (pepper spray, collapsible baton, big tree branch, firing a shot into the dirt, whatever).
But to think that you would even be able to evaluate such a scene in a school parking lot while walking by on a public street is a little too far into "fantasy what if land" to really even bother with.
And any set of adult chaperones who would let a young girl walk out into th parking lot alone at sch an event should be held criminally liable for anything that happens to her anyway...