The Supreme Court, tackling the explosive issue of school desegregation in two contrasting decisions, yesterday defined narrow circumstances under which voters may use the ballot to restrict busing.

In one case the court upheld California's Propositon 1, which restricts the ability of state courts to go further than federal courts in ordering busing. The Supreme Court said the measure was acceptable because voters followed established procedures to change state law.

But the court struck down as unconstitutional an anti-busing initiative passed by voters in the state of Washington that overruled a Seattle school board decision to begin a voluntary busing plan.

There, the court said voters removed the school board's ordinary authority over pupil assignment, disrupting established decision-making procedures, solely to ban busing. That unusual treatment constituted racial bias.

Neither case affects the power of the federal courts to order busing to remedy intentionally segregated schools.

One ruling allows Seattle to continue busing 14,000 school children as part of a desegregation plan established voluntarily by the local school board, despite the fact that 66 percent of the state's electorate voted to end it.

The other allows California's voter-approved Proposition 1 to prevent the busing of 40,000 students in Los Angeles as originally ordered by a state court which chose to go beyond federal desegregation requirements.

The contrasting treatment in two separate but related cases was the court's first attempt to rule on the constitutionality of grass-roots efforts against busing not strictly required by a federal court, such as plans imposed voluntarily by school boards or by state court systems.

The court's actions are expected to affect other voluntary programs to help minorities, such as government affirmative action plans, and the ability generally to undo remedies never actually required.

In a 5-to-4 vote the justices said Washington's major mistake was to allow the local school board to make virtually all important educational decisions, such as curriculum policy, except whether to bus for racial purposes. That created a special hurdle with a racial cast, Justice Harry A. Blackmun said, writing for the court.

It removed "the authority to address a racial problem--and only a racial problem--from the existing decision-making body, in such a way as to burden minority interests," he wrote.

Proposition 1, the court said in an 8-to-1 decision, was by contrast "racially neutral." Without violating the U.S. Constitution, Justice Lewis F. Powell Jr. wrote, it followed California's ordinary procedures of government to repeal something--a liberal school desegregation law--it never had to establish in the first place.

"Were we to hold that the mere repeal of race-related legislation is unconstitutional," Powell said, "we would limit seriously the authority of states to deal with the problems of our heterogeneous population."

The lone dissenter in the California case was Justice Thurgood Marshall, who said he saw no relevant distinction between the two voter measures. Both, he said, are burdens "in the path of those seeking to counter the effects of nearly three centuries of racial prejudice."

In Seattle, the city school board decided on citywide busing in 1978 after years of less far-reaching experiments and threats of lawsuits from civil rights organizations. Of the city's 112 schools, 26 were considered imbalanced by the board. The total school enrollment was 37 percent minority, with Asian-Americans and Hispanics outnumbering blacks.

In response, a group favoring neighborhood school enrollment successfully campaigned for a statewide measure, Initiative 350, mandating neighborhood attendance and, in effect, an end to the Seattle plan.

Blackmun, joined by Justices William J. Brennan Jr., Byron R. White, Marshall and John Paul Stevens, ruling in Washington et al. vs. Seattle School District No. 1, found the initiative flawed in important ways.

It explicitly allowed busing for some purposes, such as transporting handicapped students, but barred it for only one: achievement of racial balance. Thus, "despite its facial neutrality," Blackmun wrote, "there is little doubt that the initiative was effectively drawn for racial purposes."

In addition, it "reallocated" governmental power to achieve its ends. It bypassed the local school board, which usually makes school assignment decisions, and placed the power over busing, and only busing, at the state level.

"When the political process or the decision-making mechanism used to address racially conscious legislation--and only such legislation--is singled out for peculiar and disadvantageous treatment, the governmental action plainly 'rests on distinctions based on race,' " the justices wrote.

Powell, joined by Chief Justice Warren E. Burger, William H. Rehnquist and Sandra D. O'Connor, dissented, calling the majority ruling an "unprecedented intrusion into the structure of a state government."

In Crawford vs. Los Angeles Board of Education, the California courts ordered the busing in Los Angeles based on state court rulings requiring busing to dismantle even de facto segregation, that not brought about by government policy. That goes further than federal court requirements, which permit the imposition of busing remedies only to reverse intentional segregation.

California voters, in response, amended their constitution in November, 1979, to make state law conform with the less strict federal law. Busing, the amendment said, may not be ordered except to correct intentional segregation. Upheld by the state courts, the amendment ended the Los Angles busing in April, 1981.

Powell, joined by everyone except Marshall, noted first that Proposition 1, the referendum on the amendment, "does not inhibit enforcement of any federal law or constitutional requirement."

First, the courts in California held that the state constitution required going beyond the U.S. Constitution, Powell said. Then, the voters amended the state constitution to wipe out that interpretation. "The simple repeal or modification of desegregation or anti-discrimination laws . . . never has been viewed" as racially discriminatory, Powell said.

"Nor can it be said that Proposition 1 distorts the political process for racial reasons or that it allocates governmental or judicial power on the basis of a discriminatory principle," he said.

"Having gone beyond the requirements of the federal Constitution, the state was free to return in part to the standard prevailing generally throughout the United States."