It all gets back to teachers' unions.
Teachers CAN be evaluated, quantitatively. The students' proficiency could be tested at the beginning of each school year and again at the end. Progress can be measured. Factors can be developed to allow for differences in ability, demographics, culture, you-name-it. At the end of the process you have a number of teachers all teaching the same material to classes that are comparable, and the best and worst teachers will manifest themselves very clearly. The best teachers can be observed to ascertain what it is that makes their performance better than the others. Sometimes lower-performing teachers can be taught how to be better, but not always. A person close to me is a very enthusiastic and competent (knowledgeable in her subject) teacher, but her results are dreadful. Not everyone can teach.
But unfortunately, (1) the people in the best position to develop the evaluation system are teachers, and (2) the ONLY representatives of the teachers that Management is permitted to address are the UNIONS. And the Unions want no part of quantitative evaluation because the inevitable result of QE is the removal of the under-performing teachers, and as we all know, Unions - under any and all circumstances - work for the benefit of the worst performers.
So they constantly state that performance evaluation is impossible, and where under-performance is detected, the ONLY solution is to re-train (and never jettison) the under-performing teachers. Thus, an American public school teacher CANNOT BE TERMINATED FOR NON-PERFORMANCE. [Which is why parents and taxpayers are so resentful of teachers' compensation, benefits and absurdly-early retirement].
If a foreign power wanted to sabotage the American Public School System, to ensure that mediocrity reigns eternal, there is NOTHING they could do that would have better effect than to "organize" (unionize) every school district in the country. Doing that, they ensure that (a) there is NO INCENTIVE WHATSOEVER for teachers to strive to be the best that they can be, and (b) there is NO PENALTY WHATSOEVER for mediocre performance, year after year until early and generous retirement.
God bless the millions of kids who are able to get a good education despite the endemic problems that our schools present. The only upside is that teachers LOVE teaching the best students, and most school districts have programs in place that identify good students and allow them to be taught in Honors (or whatever) programs, segregated from the GenPop.
And "diversity" is the second worst thing that ever infected public education. A horrific waste of resources, it is a manifestation of a devotion to a "cause" that has NEVER BEEN DEMONSTRATED TO HAVE ANY POSITIVE ACADEMIC EFFECT. In fact, it harms academics by placing unqualified students in advanced programs, thus hindering the progress of everyone else.