Here is something you can use going forward, a rule of thumb: When someone claims that what they are putting forward is not an opinion but is a fact, nine times out of ten it's an opinion. There are some naive sorts who are new to the game and are genuinely trying hard to pare all opinion from their writing, or at least key parts of it. I once was one myself. It's not impossible. It's just not very usual, especially among professional writers. If they say it's not an opinion, it's an opinion.
Worse, the opinion they are trying to sell as a fact is usually the key item, the debateable point on which everything else rests. As in the linked essay, where the paragraph immediately before the declaration that he has not begun to state an opinion assumes that it is the discipline which is at fault. Which is his entire point. He is assuming what he purports to prove.
Not the students, or lead paint, or parents, or how much sleep the kids get, or the previous schools, or drug use, or nutrition, or any of a dozen other possibilities are an explanation. It's the discipline. There may even be a negative correlation between what is claimed to be not-opinion and the amount of opinion it actually relies on.