It has never been the case that serious university-level courses in, say, history or political theory or linguistics didn’t require serious paper writing.
High school is mentioned explicitly, but we aren't talking about it exclusively. "High schools imitate universities," in pg's words, and he spends rather a lot of time historicizing the issue in those terms.
(The account he offers is, well, not so much flat-out wrong as woefully incomplete. See A.J. Minnis' Medieval Theory of Authorship for a definitive scholarly account of the genesis of professional literary studies.)
I had to write essays for subjects other than English from middle school onwards. IIRC mostly for history, with some geography as well. Key point being from my education, essays have always been about making a structured argument, and not exclusively about literature.
Same for me. When I was in the equivalent of high school, I had to write essays in Art, English, History, Geography, Latin, Biology, and Physics.
History, in particular, explicitly emphasised structured arguments and critical analysis of sources. I think we wrote short essays, in class, at least 3 times a week. We had a fantastic teacher and I remember it being good fun...felt like decoding puzzles.
Another example I can remember was in English. We spent several weeks reading, discussing, and writing about Brighton Rock in a very structured way. We were then given a creative assignment with 3 options; one was to write a screenplay, which I did. So at 14/15 I absolutely loved writing my first (and only) screenplay.
Maybe I was lucky, but I enjoyed writing essays at school.
It has never been the case that serious university-level courses in, say, history or political theory or linguistics didn’t require serious paper writing.