It probably takes considerable effort to teach them to speak, read, and write formal Castilian Spanish.
I bet you are right. It’s probably like a kid from the ozarks trying to learn to learn chaucer.
When my high school in Canada started offering Spanish, the South/Central American immigrant kids took it thinking they’d have an easy class. They did poorly, and always argued with the teacher. We of course were being taught Castillian, not Mexican.