My brother teaches algebra at a black public school. The majority of his kids are seniors, yet can't multiply 6x12. Most refuse to try, a lot don't even attend class. Most come from families on welfare, therefor they decide to have kids early as instead of working/college etc, so they can continue the cycle.
Even if he fails the kids it doesn't matter, they don't care, their parents don't care. Some kids get to graduate because they have knocked up too many students already, and the school doesn't want an already 20 year old staying another year knowing he'll get more students pregnant.