SAN FRANCISCO -- A federal judge Tuesday refused to extend a long-running court-supervised consent decree governing student assignments in the city's schools.

U.S. District Judge William Alsup said the diversity index now used to assign students to schools is ineffective in preventing racial segregation and may even have increased segregation.

Alsup wrote that the system "has allowed, if not fostered, resegregation in the San Francisco schools."

The judge ordered in a 32-page ruling that the consent decree must end as originally scheduled on Dec. 31.

He turned down a request by all parties in the case - the San Francisco Unified School District, the state schools superintendent, black families and Chinese American families - to extend the settlement for another 18 months.

Alsup said policy decisions should be returned to school officials.

He wrote, "It is clear that no federal judge is as well-equipped as school professionals to make the educational policy decisions involved."

The case - described by Alsup as the oldest case on the Northern California federal court docket - dates back to a desegregation lawsuit filed in 1978 by black parents and the National Association for the Advancement of Colored People.

That case resulted in a 1983 settlement that set racial enrollment limits for schools. Alsup said that version of the settlement resulted in better integration of the schools in the 1980s and early 1990s.

But in 1994, a group of Chinese-American parents filed a second lawsuit claiming that the racial enrollment caps discriminated against their children and prevented them from attending the schools they wanted.

The second lawsuit was settled with another consent decree replacing the enrollment caps with a diversity index, which looked at several socioeconomic factors but did not consider race.

Lawyers for the school district and the parties in the case could not immediately be reached for comment. The district is currently in the midst of contract negotiations with its custodians, secretaries and cafeteria workers.