Although the No Child Left Behind Act was intended to help "all students meet high academic standards," it is focused on subgroups of low-achieving students. The authors analyze the possible impact of the legislation's requirement for performance reporting by racial subgroup in light of the considerable racial segregation in U.S. schools. In particular, using data on elementary and middle schools in New York State, the authors show that the schools are so highly segregated that more than half are too homogeneous to report test scores for any racial or ethnic subgroups. In addition, they show that the racial achievement gap is greatest across segregated schools rather than within integrated ones. The authors analyze the characteristics of schools that are and are not accountable for subgroups, finding that urban schools and large schools are particularly likely to be accountable, and conclude with implications for the reach of the law and for incentives for school segregation. (Contains 7 tables and 20 notes.)