It is not like we don't do research into effective teaching methods.
It is not also like some schools have 90% of kids above grade level.
Ex: I went to Bronx Science, a specialized high school in NYC where 98% of the students are above grade level and go to college. The principal at the time was known to be an obvious subject of nepotism and generally made poor decisions often. The guidance department was a joke. Teachers were above average because schools full of smart kids attract smart teachers, but not so amazing that they'd make up for kids who started below grade level if there were any.
My wife actually went to the same high school and got depression, she dropped out of BX Science and went to a much lesser rated school. The teachers there were actually better in many ways (more organized, better processes, really useful classes work skills and stuff were available due to grants) but the students were still generally below grade level and less than 1/3 went to college according to the stats. She was effortlessly at the top of her class even though she was severely depressed and missed days all the time.
The reason I think that is Covid. With kids learning remote and going all digital, the facade was lifted from teaching. Parents can and did directly experience what our kids were going through.
Our kids’ lowest standardized State scores were with teachers who were terrible. A science teacher who hated kids in general (really). A general teacher near retirement phoning it in. A math teacher who said “read section 5.6 in your book and do the homework” - and that was class.
Our kids did well where the teachers were creative. Where they interacted with students. Where they gave direct feedback and corrections. Where they gave learning reinforcement extra credit to help them focus in areas they had trouble.
In good public schools and many private schools, these bad teachers are not tolerated. In poor ones, the bad teachers flourish, and the kids suffer.
You'd need to compare the performances of kids with varying levels of help at home in classes with good teachers vs bad teachers.
My own experience in school was that I tuned out bad teachers completely. If the teacher was good, I engaged more with the material in class. But in all subjects I had my parents' considerable expertise to fill in any gaps. And I did well in every subject, regardless of teacher quality.
Pushing this on parents and home life is naive, immoral and wrong.
A teacher's impact is diffuse and spread over 30 students. A parent might only have an free hour to help their kid with homework. But it's still more individual attention than any great teacher can give.
Parents can motivate and enforce standards on their children in ways that teachers just cannot do. No teacher can ground you for failing a quiz, or treat you to Disneyland for winning a science fair prize. Most people would rather make their parents proud than their teachers.