As soon as Sade begins her new school in England, it's clear to her that she's going to have a lot of trouble with Marcia, the resident bully. Marcia makes it clear that she doesn't like Africans and so she singles out the new girl for bullying and harassment. Like Sade, Marcia is black, but she comes from a Jamaican, not an African background, which somehow makes her feel superior.
What Marcia's unpleasant behavior towards Sade indicates is that racism can blight society to such an extent that it can even keep people apart who belong to the same race. Marcia and Sade are both black, but there's no racial solidarity between them. Naidoo appears to be suggesting that society's prevailing racism has been internalized by too many black British people to the extent that it manifests itself in a misguided sense of cultural superiority.