A federal judge today signed an order barring the Prince George's board of education from disposing of 19 school buildings on the grounds that the schools might be needed to implement a new county desegregation plan if one is ordered by his court.

The action leaves the county free to dispose of or alter seven other schools that the board wants to use for a variety of purposes.

U.S. District Court Judge Frank Kaufman also insisted during a four-hour hearing yesterday that the desegregation case be brought to trial by Feb. 1, rather than early May, as the plaintiffs had requested. "I am very anxious not to wait to try this case any longer than necessary," he added later. "We're not going to have any surprises and secret weapons in trial."

Judge Kaufman also allowed the NAACP itself to be admitted as a named plaintiff in the case, and ruled that three new couples could be substituted for couples in the original busing case who no longer wanted to participate.

Most of today's hearing centered on the disposition next year of the Frederick SasscerJunior High School, which the board of education wants to close and use to consolidate its administrative offices. Two of the buildings presently used by the board are slated for demolition to make way for a new courthouse in Upper Marlboro. The board insisted that planning for the change could not be delayed until the conclusion of the trial.

Lawyers for the NAACP said the school was "relatively racially stable" and would be an important factor in any final desegregation plan. Lawyers for both sides finally agreed to allow the board to close the Sasscer school and, if a school is needed in that area to fulfill a future desegregation plan, to reopen the now-closed Upper Marlboro Elementary School instead.

Attorneys for the school board were elated by Kaufman's decision to exempt seven schools from his order. "We consider it a major move because it allows the school system to continue its current operations," said school board spokesman Brian Porter.