I have a question, Rude... where did all the other children in the school district go? What I mean is... of course the difference between blacks' and whites' scores would diminish IF only the motivated blacks and whites attended a school. Duh (not intended disrepectfully). The real acid test is when ALL the kids attend private schools.
Americans have had home and private schooling before you know... we called it "the 1800's". Some of the smartest people came from the South, where private and home schooling gave us some of the best scholars of the age. Too bad 98% of the population went uneducated, but look at the results those private schools and home schooling produced.