As concerns mount over the resegregation of the nation’s public schools, a new federal study shows that black and white students at schools with a high density of black students perform worse than those at schools with a lower density of black students.

The report, released Thursday by the National Center for Education Statistics, the research arm of the U.S. Department of Education, sheds new light on the achievement gap between white and black students and bolsters policymakers’ fears about the ramifications of increasingly segregated schools.

"I think that we all have some sort of anecdotal sense that racial isolation or the resegregation of schools going in that direction is not a good thing,” says acting NCES Commissioner Peggy Carr. “It’s not good for anyone. But being able to define it and put your finger on it … and be more diagnostic about the probable impact was really eye-opening for me.”

The report found that, on average, white students attended schools that were 9 percent black while black students attended schools that were 48 percent black.

Achievement was lower for both black and white students in schools where black students accounted for more than 40 percent of the student body, compared to schools where black students accounted for less than 20 percent of the student body.

Those findings weren’t entirely unexpected. But what did surprise Carr, she says, was that the achievement gap for black students was largely due to the performance of black male students, not black female students.

Further, Carr explains, black males actually did worse in schools with a high density of black students while white males did better, compared to schools with lower densities of black students.

“Even when we account for factors associated with higher achievement such as student socioeconomic status and other student, teacher, and school characteristics, we see that black male student achievement is lower in schools with higher percentages of black students,” Carr says.

The black-white achievement gap has been studied for years, but its relationship to school composition has generally not been explored.

The study was conducted using data from the results of the 2011 eighth-grade math test given as part of the National Assessment of Educational Progress, an assessment that’s given to U.S. students in various subjects in grades four, eight and 12.

Civil rights groups were quick to point out that decades of inequitable funding for schools, particularly for those with large populations of minority students, has played a significant role in establishing achievement gaps.

“In this country, education resources are not distributed fairly and our inequitable results bear that out,” says Liz King, director of education policy at The Leadership Conference on Civil and Human Rights. “It’s time we got serious about school diversity and providing resources necessary for every child to succeed.”

Carr agrees.

“I think this [report] is something that states can use if they want to be more thoughtful about how to direct their resources and where to look to try to conquer these stubborn gaps,” she says. Closing achievement gaps within schools, she added, could be fixed, in part, by increasing access to things like technology, updated textbooks and qualified teachers.

The groups also underscored, however, that just because a school has a large density of black students, doesn’t mean that it can’t be successful on its own.

“We also have to highlight that there are examples of schools that have an overwhelming number of African-American students and are excelling at high levels, and I don’t want that point to get lost in our effort to push for more equitable resources,” says Sonja Brookins Santelises, vice president of K-12 policy and practice at The Education Trust, a civil rights advocacy organization.

“We still need to advocate for those situations to be improved and equalized, but we can’t make that an excuse,” she says. “We can’t say that until we have full integration that we cant expect black children to achieve. That would take us down a very dangerous path.”