One of the reasons - probably one of the biggest reasons - for continuously lower test scores in America is the quality of the care these children receive.
Over 40% of children in the US are born to unwed mothers, compared to 5% at the beginning of last century. About 70% of unwed childbirth is by black mothers, versus just 24% in 1965. The last two peaks of teenage pregnancy were 1991 and 2007 -
so the children of teens have more children, whom are now 10 years old, with 26yr old parents, and whose grandparents are about 42yrs, with great grandparents about 70 years old.
The financial and social strain on such families is intense. It's also difficult to be the best parent you can be when you yourself are still not an adult. And it virtually guarantees poverty for these children. There is almost no reasonable assistance given to the families of these children - assistance like food, shelter, access to education, daycare, transportation.
Kids are getting left behind not by their school system, but a social system that has ignored them as part of a lower class of society that we'd rather starve and struggle than give assistance to.
If you don't believe this, go into a [public] school in Philly, Baltimore, New York, and ask about the grade level of reading comprehension of the kids, what their family life is like, and their economic status. You will see a trend in all three: "poor."