A child coming from an unmarried mother, regardless of race, is 3 times more likely to be held back in school or flunk out. Around 25% of white children are illegitimate, 80% of black children are. Simply on the basis of three times as many children born out of wedlock, without a stable married family to reinforce academics and maintain discipline, you get much worse average academic achievement.
If you don't have kids out of wedlock, don't marry before the age of 20, finish high school and work full time, the odds of being in poverty are single digits. Poverty can lower IQ due to malnutrition and stress. Due to most blacks not being married when they have kids, you have a majority in a stressful situation of high poverty - and worse - entire communities where there are almost no married stable families to stabilize the area. High stress, decreased intellectual development.
There is a very bad ethos in some black communities that acting white is immoral. The kid who gets As versus Bs and Cs is beat up for "acting white", as is the one who expresses an interest in chess instead of sports. This tears down the high acheivers while reinforcing a culture of low expectations.
When a family is in poverty, the extra couple hundred a month from Social Security for a child with an official learning disorder is golden. And parents seek out special needs certifications for the kids to get the money and then keep them there - this isn't racial, there were poor whites in Appalachia who pulled kids out of literacy classes to make sure they didn't improve so much they weren't learning disabled and the extra SSDI money stopped. But the same trend is likely occurring in poor black communities.