Lawyers for the Prince George's County school board told a federal judge in Baltimore yesterday that the NAACP does not represent blacks in the county and asked him to dismiss the civil rights group's motion to reopen the 1972 school desegregation suit against the board.

In a motion filed with U.S. District Court Judge Frank Kaufman, the board denied the NAACP's allegations that the county school system has not been desegregated and suggested that if the group wanted to make that argument, it should do so in an entirely new court suit.

Although NAACP lawyers have been involved in the suit since it was filed in 1972, they did not seek to enter the case as plaintiffs until Sept. 1, when they asked Kaufman to reopen the case on the grounds that the past nine years of busing have not adequately desegregated the schools.

The original suit was filed by eight black parents on behalf of all blacks in the public school system. Since then, according to the school board's motion, six of the eight plaintiffs have lost their standing to represent black children.

Former county NAACP president Sylvester J. Vaughns, whose name led the list of plaintiffs in the original suit, has said that he does not see the purpose of reopening the litigation, but would not seek to stop the latest effort by the NAACP.

Paul Nussbaum, the school board's attorney, said that both Vaughns and a second plaintiff in the controversial suit "have publicly stated that they were satisfied with desegregation or that they opposed the suit." Four other original plaintiffs no longer have children in Prince George's schools, according to Nussbaum.

As for the the remaining two plaintiffs, Nussbaum said, "We do not consider them a class based upon the voices of other blacks in the county."

Nussbaum added that NAACP cannot claim to represent blacks as a class in the county because of public disagreement over busing within the black community. It also cited a 1979 agreement by William Martin, then NAACP president, and school board member Norman Saunders to reduce busing for integration. Martin was subsequently ousted from his position when the agreement became known.

Officials of the NAACP could not be reached for comment last night.

In its motion two weeks ago, the civil rights group charged that nearly half of the county's 216 schools were "racially identifiable" in 1980, almost as many as before the court order to desegregate. The NAACP defines racially identifiable as any school whose black population varies by more than 20 percentage points from the percentage of blacks in the schools as a whole. The school system was 49 percent black last year.

School officials maintain that the increase in the number of predominantly black schools is due to expansion in the percentage of black students in the schools from 20 percent in 1972 to nearly 50 percent. This growth parallels a rise in the county's black population from 13 to 37 percent in the last decade.

Critics have charged that the court ordered busing plan is reponsible for the massive departure of white families from Prince George's in the 1970's, particularly from the central areas of county most affected by the plan. But NAACP leaders say that it is possible to achieve a greater degree of integration by including a number of predominantly white schools in the outer areas of Prince George's without necessarily increasing the number of children on school buses.