In what civil rights leaders called a major victory for school desegregation in the North, the Supreme Court yesterday sustained its position that federal judges can order sweeping desegregation plans for an entire school system where school board policies have led to racial imbalance in only a part of that system.

The court upheld such a system-wide desegregation plan by a narrow 5-to-4 vote in the case of Dayton, Ohio, where some 18,000 students are already being bused. And in a Columbus, Ohio case, the court confirmed that school board policies that led to isolated instances of segregation had an effect on the whole system. This decision cleared the way for massive busing in September.

In both cases, the court upheld its 1973 decision, Keyes vs. School District 1 of Denver. It ruled then that if a federal judge finds intentional segregation by school officials in a "substantial portion" of a system, it will be presumed that official policy-and not housing patterns-created segregation everywhere in the system, unless school officials could prove otherwise.

By placing the burden of proof upon school officials, the court in effect made it easier for civil rights groups to bring charges.

In yesterday's majority opinion, Justice Byron R. White said the court did not perceive any misuse of Keyes.

Justices William H. Rehnquist, Lewis F. Powell, Potter Stewart and Chief Justice Warren Burger dissented in the Dayton case. Rehnquist and Powell dissented in Columbus.

Rehnquist called the Columbus decision "as complete and dramatic a displacement of local authority by the federal judiciary as is possible in our federal system." He compared his colleagues to Pontius Pilate, washing their hands of the "disparate results" of their decision. He also accused them of becoming "Platonic guardians," unsurping the neighborhood school system.

"Whether the court's result be reached by the approach of Pilate or Plato, I cannot subscribe to it," Rehnquist wrote.

Yesterday's decisions reaffirmed the court's position that communities which abolished all mandatory segregation laws after the 1954 Brows v. Board of Education decision still had a responsibility to move to wipe out the effects of past segregation. They upheld lower courts' positions that a failure to act decisively to eliminate vestiges of past legal segregation amounted to a current segregation policy.

The decisions may end the continuing argument of busing opponents that segregated schools are caused mainly by housing patterns and not deliberate school board policies.

The Columbus decision removes the final road-block to a busing plan that was postponed before implementation last year by Justice Rehnquist. About 7,000 pupils will be bused under the plan as originally drafted by federal district Judge Robert Duncan.

Duncan will review his plan to see if any alterations are need before September implementation, an aide said.

In Columbus, local leaders interviewed by telephone said they believed yesterday's decision would provide new impetus for a constitutional amendment prohibiting busing.

Rep. Samuel L. Devine (R), who represents part of Columbus, said "You can't appeal a decision of the United States Supreme Court, but I don't think the people are going to lie down and play dead. I think they're going to go to the last resort, and that's a constitutional amendment."

In Dayton, School Superintendent John Maxwell said in a telephone interview the existing plan buses about 20,000 students a total of 14,000 miles a day, and it "is killing us. Nobody wants it, not the blacks or the whites. Looks like we're dead unless we get a constitutional amendment."

In other desegregation cases, the court yesterday upheld busing plans for Austin, Tex.; Louisville, Ky., and Minneapolis.