Don't see why this notion, as such, is inherently bad.
There is a lot of great writing in histories, biographies, etc.
The problem, however, is likely to be not the fiction/nonfiction issue. It's the books they will approve in both categories. No dead white males need apply.
You’re right that it’s not a fiction versus nonfiction thing. Literature, after all, is only one subject. I don’t know about the percentages, but the rest require a different sort of learning. The problem is the paucity of higher culture that will get into students’ heads no matter the subject.
If we wanted to go another way, for instance turn high schools into trade schools, I could see that. But then I’d wanna channel those destined for college into a seperate course. As it is we’re not going to give low- or middlebrow kids marketable skills, and we’ll churn out doctors, lawyers, and scientists who can’t think. Which is to say nothing of all the loafabout English or philosophy majors who can’t be trusted to know anything about English or philosophy, even.