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.