At Columbia High School in Maplewood, New Jersey, top-level classes are almost all white; low-level classes are almost all black. In-school segregation is common at schools that track students by performance. From the New York Times:
Columbia High School seems to have it all – great sports teams, great academics, famous alumni and an impressive campus with Gothic buildings. But no one boasts about one aspect of this blue-ribbon school, that its classrooms are largely segregated.
Though the school is majority black, white students make up the bulk of the advanced classes, while black students far outnumber whites in lower-level classes, statistics show.
. . . Educators say that leveling allows smarter students to be challenged while giving struggling ones the special instruction they need. But many students, especially those in the lower levels, which often carry a stigma, say such stratification makes the rocky adolescent years only harder.
Stigma isn’t really the issue. The problem is that students live down to low expectations. But often the low-track students really do need to be taught skills and knowledge that other students have mastered already. And the good students will leave if they can’t take challenging college-prep classes that not everyone can handle.
As Eduwonk says, this is the reason that No Child Left Behind requires schools to break out scores for subgroups, instead of averaging scores for the school as a whole. It’s possible for students to be doing fine on average while some groups are doing poorly.