The editorial quotes from Justice Thomas' concurrence:
Meanwhile, Justice Clarence Thomas, in a concurring opinion, took about half a line to say, "I agree," and proceeded to write one of the most compelling essays I've seen on the decline and fall of American public education. I would happily hand out Justice Thomas's opinion on street corners (though www.supremecourtus.gov relieves me of that burden).
What he's done is rummage back through school cases, mostly from 19th century state courts, to invoke the idea of a public school. His premise is that the schools' role was most certainly in loco parentis, in that they and parents broadly agreed on what made an adolescent grow into a good person; what schools need least is court interference in this hard job.
The 'in loco parentis' phrase is what can frighten parents so much.