Points taken....
The kids would have to be monsters on a scale beyond even the Columbine killers in my opinion, as the school context and multiple targets would allow the latter to depersonalize what they were doing. Not so with shooting these little(r) girls point blank.
Of course, no reason to assume that only kids would be in this situation. Everything you say could be true and it still not necessarily be very young males. Certainly 18-19 yo's may be able to kill like this, and still for less motivation than the crime seems to indicate to us.
One reason I lean toward something bigger, and outside the local community, is that I think a local shooter would have been id'd already. Like, if it were kids, they'd be found out pronto, not have it be like this.