Cases from Seattle and Louisville, Ky., brought the divisive issue before the court for the first time since 2003, when a 5-4 ruling upheld the limited consideration of race in college admissions to attain a diverse student body.
A decision against the school districts could imperil similar plans in hundreds of districts nationwide and leave public school systems with a limited arsenal to maintain racial diversity. A ruling is expected by next summer.
In Monday's cases, parents sued after their children were denied admission to the schools they preferred because of their race. The school policies in contention were upheld by federal appeals courts and are designed to keep schools from segregating along the same lines as neighborhoods.
"These are two big cases because school districts all over the country in one form or another use race as a factor, but certainly not the only factor, in determining which kids go to which schools and for what reasons," said CBS News legal analyst Andrew Cohen said.
Cohen adds that "everyone will be watching, too, to see how the newest justice, Samuel Alito, handles his first big affirmative action case."
Justice Anthony Kennedy, who could hold the decisive vote as the swing justice since Sandra Day O'Connor's retirement, joined his conservative colleagues in expressing deep skepticism about the programs.
School districts court trouble by "characterizing each student by the color of his or her skin," Kennedy said during the argument over the Seattle case. "It seems to me that should be allowed, if it's ever allowed, as a measure of last resort."
Lawyers for the parents and the Bush administration said the plans violate the Equal Protection Clause of the 14th Amendment, enacted after the Civil War as part of the effort to remedy the effects of slavery.
The issue presents a unique challenge to Louisville's schools, which spent 25 years under a court order to eliminate the effects of state-sponsored segregation. The Jefferson County, Ky., school board, which encompasses Louisville, decided to keep much of the court-ordered plan in place to prevent schools from re-segregating.
"What's constitutionally required one day is constitutionally prohibited the next day? That's very odd," Justice Ruth Bader Ginsburg said, sentiments shared by her three liberal colleagues.
In a reversal of sorts, the court's liberals spoke in favor of local control of education policy.
Francis Mellen Jr., representing the Louisville schools, called the plan a success story that enjoys broad community support, including among parents of white and black students. Joshua McDonald, the child whose situation as a kindergarten student led to the lawsuit, was admitted to his school of choice beginning in the second grade.
Still, Kennedy wanted to know whether a school system that succeeds in freeing itself from court supervision can use a student's skin color as a basis for assignment.
"We've never said that. That takes us on a very perilous course," he said.
Kennedy's line of questioning indicated to some civil rights advocates at Monday's session that their side would have a hard time prevailing in these cases. "It's even more difficult to win these cases now and it was never easy," said Theodore Shaw, director counsel of the NAACP Legal Defense and Education Fund.
Chief Justice John Roberts and Justices Samuel Alito and Antonin Scalia also expressed concern about the school plans. Justice Clarence Thomas asked no questions, but he has consistently voted against racial preference plans.
Scalia derided the school systems' policies as a "whatever it takes" approach that improperly classifies people on the basis of race.
Attorney Michael Madden, representing the Seattle school district, said race is but one factor, that it is relied on only in some instances and then only at the end of a lengthy process.
Madden drew a distinction between the Seattle school program and the subject of the court's 2003 decision, which narrowly approved the University of Michigan law school's affirmative action admissions program.
"This is not like being denied admission to a state's flagship university," Madden told Roberts. The Seattle students are "not being denied admission, they are being redistributed."
Seattle suspended its program after parents sued.
Outside the court, affirmative action supporters bearing "Fight For Equality" placards marched on the sidewalk in a brisk wind. A parent-teacher group from Chicago and several civil rights groups were among those sponsoring the demonstration.
Demonstrators chanted "Equal education, not segregation" and "We won't go to the back of the bus, integration is a must." Among the crowd were representatives of the National Organization for Women, the NAACP and students from Howard University.
Though outnumbered, there were some in the crowd from the other side.
"Regardless of how well-motivated, allowing the state to engineer racial mixing only creates racial stereotypes and increases racial tension," said Terry Pell, president of the Center for Individual Rights, a public interest law firm. "The court needs to put an end to state-mandated tinkering with race."
The Bush administration took the side of the parents who are suing the school districts, as it had intervened on behalf of college and graduate students who challenged Michigan's affirmative action policies in 2003.
Solicitor General Paul Clement, the administration's top Supreme Court lawyer, said the Louisville and Seattle plans essentially used illegal racial quotas.
Clement said school districts have an unquestioned interest in reducing minority isolation but only through race-neutral means.
That comment provoked Justice David Souter to remark that school systems "must hide the ball," seek racial diversity without relying on race.
"The whole point is to achieve a value which comes from mixing the races," Souter said.
The cases are Parents Involved in Community Schools v. Seattle School District No. 1, 05-908, and Meredith v. Jefferson County Board of Education, 05-915.