THE PRINCE GEORGE'S County School Board seems to be moving toward another appeal of the desegregation case. A vote is to be taken this week, and there's reportedly a narrow majority in favor of appealing. It's a thoroughly bad idea.

Another appeal would only undermine the board's own plans for compliance with the federal court's order. The board has decided to try to improve the racial balance of the county's schools by developing magnet schools sufficiently attractive that they can draw students voluntarily from distant neighborhoods. That requires an atmosphere of good will and good faith that will be badly eroded if the board insists on continuing its futile challenges of requirements that most other American school districts have long since accepted.

Maryland schools were once segregated by law. Where public authority once enforced segregation, the Supreme Court has repeatedly held, it has a duty to remove "root and branch" every vestige of that segregation. The board has been arguing that it did its best to comply and, if it never entirely succeeded, the fault lies with demography and shifting patterns of population. The courts have never accepted that defense for schools previously segregated by law, since population patterns are affected by the character of the schools. There are a dozen schools in Prince George's that were predominantly black in 1972 when this suit began and are still predominantly black.

The precedents are quite clear. Why do some of the board want to try another appeal? Because they nourish the hope and expectation that a conservative Supreme Court will retreat from earlier decisions. But the pivotal case on desegregation compliance, Swann v. Charlotte-Mecklenburg, was written in 1971 by Warren Burger, then and now cief justice, for a unanimous court of nine judges. Five of them still are on the bench. The Swann principles have been repeated many times by the court, notably in 1979 -- and eight of those judges are still serving.

The Prince George's school board has done many decent and constructive things over the past 13 years, but it has consistently misinterpreted the law of desegregation. When it fought the original 1972 suit in the district court, it lost. When it went to the appeals court that year it lost. When it appealed to the Supreme Court in 1973, the court refused to hear the case. When the plaintiffs came back to the district court in 1981 complaining that the court's orders were being violated, the board fought it again and, after an extremely expensive two-month trial, lost again. It went to the appeals court again and, last March, lost again. Wouldn't you agree that the message is clear? But now some of the board want to take still another appeal at a further cost in money, time and lost opportunities for the county's children.