Where I grew up (back before kindergarten was part of the public schools), after integration we had a grade 1-2 school, a 3rd grade school, a 4-5 grade school, a 6th grade school, a junior high, and a high school. Before that, the white community and the black community had neighborhood elementary schools.
The K-8 configuration might work in D.C. if the schools are strong neighborhood schools, with lots of parental interaction and tough discipline standards. If the children feel they belong to a community at the school, the extra support may help.
I'm leery, though.