I think what you're referring to has to do with the fact that our public school system is not [primarily] about education.
The goal of the left has always been to dumb down the students.
Because strictly speaking, it's not. The goal of public school educators enunciated by John Dewey more than a century ago was "to make good citizens" out of students.
That leads to the question: what is a good citizen? Is a good citizen one who excels in school subjects like english, math, geography, science or one who will do what his progressive "betters" expect him to do?
More progressives like Dewey began to infiltrate the school system and indoctrinate the students. Education in the right subjects was only part of it. The big progressives wanted obedient little progressives who would follow what the big progressives wanted. They have largely achieved their goal.