I have several teachers in the family, every one of them fantastic, really cares about their students and does the best they are able to do. Spending less certainly won't help.

The real problem is that you are giving them moldy tattered cloth and expecting them to paint you the Mona Lisa. The general exception is affluent suburbs with professional parents who value education and can support their kids appropriately.

Outside of the affluent suburbs you have too high a proportion of kids without good nutrition, without good role models and good support from a stable home life and parents who value education.

I've seen it up close and personal as my family works in a mixed district. The kids with good parents and good support reliably do much better than the rest. The opposite is also almost always true. It's a failure of a large segment of our society. It's much harder to make a masterpiece out of moldy torn cloth, times millions of students.

Private school is populated primarily by students with parents that by their basic foundational nature value education and are far more likely to provide a stable home and sufficient support.