This problem is generations deep and nationwide.
The educated class in DC send their kids to private schools, or, if they send their kids to public school, they live in the wealthy suburban counties of VA or MD. That leaves behind only the children of the underclass who are themselves undereducated and often illiterate.
This is no different than any other major city, but in DC it is ONLY the city kids who are counted, whereas, say, Detroit gets mixed in with Grosse Point for a state total. If you took DC and its surrounding suburbs you would have a far higher 'average' but still same kids with the same scores and the same problems.
.... and the cycles continue.
Here in Birminham, the school system is a baby sitting and free food service for all. We feed the future brood mares and the future drug pimps well, so that their parent (singular) can have a lot of extra dollars left of their welfare monies. The brood mares keep the cycle going and the drug pimps land in prison. They are fed very well in prison also, most likely better than school. We had a federal judge here that his son was on free lunch the whole time he was in school, go figure.