Yeah, they are kinda new in the way that they’re a goal, not a one-off standalone act of petty revenge. Nobody trended blowing up schools after that. After a quick look at wikipedia for a list of school massacres we had less than one per decade after the 1927 event, then a half-dozen in the '90s, now 21 or so since '00.
So I don’t know that I agree that they’re “nothing new” because that’s such a grim and arbitrary undefined way to put it. How few would there have to be to make them new by the context of the specified 1927 event?
I think I agree with the person you responded to - partly…because there was already an uptick in school killings in the '90s, however the Columbine massacre certainly sits on a point that marks a definite change from a half-dozen in a decade to a solid ten per decade, or at least one per year, and the apparent use of schools as a target in the eyes of the killers.