I get frustrated with some of the big "anti-racism" campaigners; they strike me as making the smaller issues big ones and causing divisions all over again.
On the specific issue I think it's not so much because the students are black they have this attitude - just because they are in a minority and have the anti-majority sentiment. I saw a similar problem at my school when girls where let into the all male establishment for the first time. Even though girls tend to do better academically at that age in our school they did worse and had higher rates of no homework completion/detention (as I recall). Im guessing for very similar reasons.
To be honest though I think parents are the main problem; and not just in minority groups now. Even my Aunt who is in her early 40's has no interest in her kids schooling - and they suffer for it. The current parental generation seems full of similar attitudes - my mother is a teacher and she says getting parents to take an interest, encourage learning out of school and other activities is the biggest challenge she faces at the moment.