WASHINGTON (AP) – The Supreme Court said Monday it will decide the extent to which public schools can use race in deciding school assignments, setting the stage for a landmark affirmative action ruling.

Justices will hear appeals from a Seattle parents group and a Kentucky parent, ruling for the first time on diversity plans used by a host of school districts around the country.

Race cases have been difficult for the justices. The court’s announcement that it will take up the cases this fall provides the first sign of an aggressiveness by the court under new Chief Justice John Roberts.

The court rejected a similar case in December when moderate Justice Sandra Day O’Connor was still on the bench. The outcome of this case will turn on her successor, Samuel Alito.

“Looming in the background of this is the constitutionality of affirmative action,” said Davison Douglas, a law professor at William and Mary. “This is huge.”

Arguments will likely take place in November. The court’s announcement followed six weeks of internal deliberations over whether to hear the appeals, an unusually long time.

In one of the cases, an appeals court had upheld Seattle’s system, which lets students pick among high schools and then relies on tiebreakers, including race, to decide who gets into schools that have more applicants than openings.

The lower court decision was based in part on a Supreme Court ruling three years ago, written by O’Connor, which said that colleges and universities could select students based at least in part on race.

The court also will also consider a school desegregation policy in Kentucky. That case is somewhat different, because the school district had long been under a federal court decree to end segregation in its schools. After the decree ended, the district in 2001 began using a plan that includes race guidelines.

A federal judge had said system did not require quotas, and that other factors were considered including geographic boundaries and special programs.

A mother, Crystal Meredith, claimed her son was denied entrance into the neighborhood school because he is white. The Jefferson County school district, which covers metropolitan Louisville, Ky., and has nearly 100,000 students, was ordered to desegregate its schools in 1974.

The court will also consider whether Seattle’s so-called integration tiebreaker system, which has been discontinued, is tailored to meet a “compelling interest” by the school.

A group called Parents Involved in Community Schools sued in July 2000, arguing that it was unfair for the school district to consider race, and Seattle halted the system.

Lawyers for the Seattle school district had told justices that it was not known what the district’s new school board and new superintendent would do now.

Under the district’s plan, the first tiebreaker was whether an applicant has a sibling already at the school. The second tiebreaker was race: which applicant would bring the high school closer to the districtwide ratio of whites to nonwhites, roughly 40 percent to 60 percent. The third tiebreaker was distance, with closer students getting preference.

Seattle has about 46,000 public-school students. The racial tiebreaker helped some whites get into predominantly minority schools, and vice versa.

The cases are Parents Involved in Community Schools v. Seattle School District, 05-908, and Meredith v. Jefferson County Board of Education, 05-915.