The one caveat to take from the claims in this - most of which I agree with - is that not all students are equally teachable, equally studious, equally diligent nor of equal intelligence.
As much as it would be nice to think that teachers of equal worth can teach a class of the same number of students and those students produce equal academic results is simply not reality. I heartily believe that teachers comprise no more than 1/2 the effort in attaining the academic result of any student and by that I mean that student’s potential which is not 100% the same as another student’s. I would also add that while all teachers are also not equal, not all students put in their half of the effort to get an education.
In sum, while I think there is a lot wrong with K-12 education - in the institutions, many teachers, and the curriculum & teaching methods, improving all of that will still find some great portion of students who will not do above the median and an equal number who will never perform as well as their potential, due to no fault of the teachers.
I think there is a lot wrong with K-12 education, but a lot what is wrong with K-12 academic results is only partly due to what’s wrong with education. A good part of the problem is in society itself, a good deal of which education institutions will not solve.