As a born, raised and currently living in Spain, I had never heard that. We have plenty of classical author required in high school: Cervantes, Quevedo, Benavente, Bécquer, Machado, Lorca, Pérez Galdós... Most modern and cool teachers also recommend contemporary authors like Ana María Matute, García Marquez or Jordi Sierra i Fabra. The foreign author most recommended has to be Roah Dahl, Michael Ende, Jules Verne or Exupéry.

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.