U.S. District Court Judge Frank A. Kaufman disagreed sharply today with the contention by the Prince George's County schools that his 1972 order to desegregate did not obligate the system to prevent black schools from getting blacker.

Paul Nussbaum, lawyer to the school board for 22 years, said, "We feel that our basic duty was to not do anything to resegregate." Nussbaum said the board could show it had taken no resegregative acts, but it could not demonstrate that it always acted affirmatively to enhance desegregation.

The judge responded that "there was an obligation on the part of the school system to take acts, or not to fail to take acts, to prevent slippage or resegregation."

The exchange came as Nussbaum and cocounsel Gerson Mehlman attempted to explain why some schools in the 116,000-pupil system -- 13th largest in the nation -- have black enrollment in either far less or far greater numbers than the countywide average of 52 percent.

The NAACP, which brought the civil suit, complaining the board has not carried out Kaufman's sweeping order to end dual school systems in the sprawling county, contends that black students deliberately are segregated in many county schools. The school administration contends the racial makeup of individual schools is determined by ever-changing housing patterns, over which it has no control.

"The law is certainly not written in clear black-letter terms in this field, that's why I'm down the middle on this case," said Kaufman, who stressed that his mind is open to whether the school board is at fault.

Nussbaum told the judge that if the board had to show that it continued to act to prevent resegregation, he might need more time to prepare the defense, perhaps by rearranging the presentation of witnesses.

Earlier, in response to a question from the judge, school staffer Blair Overton said that while the board often took actions that aided integration, he knew of several instances in which it rejected staff proposals that would have lowered the black percentages at some schools and increased integration at others.

One such proposal, in 1976, Overton recalled, involved Dodge Park Elementary School in the Kentland area, which was overcrowded. A staff solution proposed busing 120 students from Dodge Park, which was 70 percent black, to Kenilworth, a Bowie school that was 17 percent black. The change would have increased the black enrollment at Kenilworth to 29 percent and lowered it at Dodge Park to 67 percent, but the school board rejected the change at a meeting on April 29, 1976. Overton said an "overwhelming outcry" from black Dodge Park parents caused the board to reject the change.

A memorandum to the board on the subject from Charles O. Wendorf, then head of the pupil accounting department, made no mention of any outcry. The memorandum also warned that reassigning pupils to two nearby predominantly black schools would have an adverse effect on the racial balance. Nevertheless, under a "temporary housing arrangement," Dodge Park students were transferred to those schools, Kenmoor and Columbia Park, in 1978.

Overton said the changes increased black enrollment at Kenmoor from 65 percent to 75 percent and at Columbia Park from 59 percent to 63 percent.

Under Kaufman's original 1973 guidelines, every school in the system was to enroll between 10 percent and 50 percent blacks. By 1978, blacks comprised 44 percent of pupils in the county.