I never heard of anybody who has read Machiavelli, Shakespeare, Tolstoi, Dante or any other "advanced" foreign author in high school.
There are some links with some "obligatory readings" for high school: https://www.edu.xunta.gal/centros/ieslamascastelo/system/fil..., https://www.educa2.madrid.org/web/lengua-castellana-y-litera... http://iesparquegoya.es/files/lengua/Libros%20de%20Lectura%2... or https://iesalgarb.es/wp-content/uploads/sites/25/2023/07/1.-... (first links when I search "lecturas recomendadas ESO") where you will find exactly one non-spanish writting author: Ray Bradbury's 451. A spicy teacher, for sure.
Maybe your coworkers read The Prince, but that is not a general recommendation or even something you heard from time to time.
The difference in Galicia is that we also have "Literatura Galega", that requires us to read Castelao, Neira Vilas, Rosalía, Murguía, Otero Pedraio... I bet other Comunidades with their own language also recommend local writters.
Unless they went to some kind of private school, where they found a weird literature proffesor with political inclinations (the kind that also recommend Gramsci or Rothbard like it is "literature"), or maybe the requirement was from a philosophy or a history class, I never heard such a thing. Not saying that they are lying, but it is not a common recommendation, even less a requirement.
I don't mean that non spanish authors are worse (I dislike many of them because they were forced to me too early, when I preferred Dahl or Tolkien), I am just saying that it makes no sense to recommend Shakespeare instead of Cervantes in Spanish Literature class, where they recommend/require books. I can't imagine Russian Literature teachers requiring "Episodios Nacionales" authored by Galdós, a book about spanish history/politics in the XIX century over any russian author about any russian theme.