This is the dirty little secret BOTH the left and right try to cover up. The leftists want to cover it up because worse *average* performance means more money for their programs. The right wants to cover it up because it undermines their agenda of privatization of the public schools (*including* the good suburban districts!)
You can do the same thing with test scores for *non-English speaking* immigrants. Of COURSE school districts like Los Angeles are going to have terrible performance on tests - the vast majority of their students can't speak or read English because of illegal immigration! In my state of Missouri, we have entire rural school districts that have been swamped with enormous numbers of Mexican immigrant children who can't speak a word of English, and yet they have to sit in the same classrooms and take the same tests as everyone else.
There are many problems with public schooling, but public schools are NOT responsible for: failure to enforce immigration laws; 14-year old crackhead mothers who abuse their babies to the point the kids are mentally disabled; parents who ignore their kids entirely, don't read to them, don't teach them proper English, etc. These "lower test scores" are showing the demographic cracks in American society, not an overall "failure" of public education.
So, what you're advocating is tougher immigration laws, fewer teenage mothers, stronger families, and better parenting. All good ideas. Unfortunately, the Left has historically opposed these ideas as well.
The vast majority of government teachers have endorsed every tax raising, family hating, homosexual loving, immigration encouraging, criminal coddling Democrat who has run for public office for as long as I can remember.
One quarter to one third of the delegates to the DNC nominating conventions are government teachers. The NEA spends over $70 million a year (against IRS code) supporting liberals.
Government teachers are not only responsible for wrecking public schools, they are responsible for many of the social pathologies outside our schools.
Minority students to not have to remain at the bottom of the pool of students. I have said time and again that a large portion of the problem with education in this country today is corrupted curriculum.
We need to go back to basics (you know, evil rote learning) and jettison the social agenda stuff that takes away valuable time in the classroom. Add teacher testing and merit pay for teachers who perform well (i.e. their students learn something) and putting educational dollars on the backs of students to go to whatever school they choose; and, we might make a dent in this problem.
However, none of this will happen because education (like just about everything else in this country) has been so politicized that it can't be touched.