In a case involving the Montgomery County schools, the Supreme Court ruled yesterday that parents of special-education students disputing proposed instructional plans for their children have the burden of proving why the plans are inadequate.

The case, brought by a Potomac couple displeased with an education plan for their son, had been closely followed by educators and parents across the nation. The 6 to 2 ruling maintains Montgomery County's practice of putting the onus on parents, which is the practice in most states. But the decision could overturn the District's rules, which had required the school system to prove the adequacy of its special-education plans when challenged by parents.

Parents had argued that school systems were better suited to bear the burden of proof in disagreements about special-education plans because they have more resources and information. But public school educators said that parents challenging a plan should have to prove the schools were wrong and worried that a change would cost millions of dollars in new litigation.

"Absent some reason to believe that Congress intended otherwise," Justice Sandra Day O'Connor wrote for the majority, "we will conclude that the burden of persuasion lies where it usually falls, upon the party seeking relief."

A federal law, the Individuals With Disabilities Education Act, guarantees disabled students an education tailored to their individual needs -- and gives families a right to a formal hearing before a neutral decision maker if they believe school officials have not come up with a good enough plan.

Jocelyn and Martin Schaffer of Potomac felt an education plan for their learning-disabled son Brian was unacceptable and argued that it was up to the Montgomery County public schools to defend it. They sought $17,000 in tuition reimbursement for the year Brian spent in a private middle school after disagreeing about his education plan.

In practice, the vast majority of litigation in special-education cases is initiated by families seeking changes to individualized education programs, known as IEPs, proposed by the schools.

Although the court's ruling maintains the current system in Maryland and Virginia, the District said it will change how it does business. The existing D.C. regulation putting the burden of proof on the schools is no longer operative, and the burden of proof is now on "the aggrieved party," said Traci Hughes, spokeswoman for D.C. Attorney General Robert J. Spagnoletti.

The decision was one of the most important education cases to reach the Supreme Court in recent years.

The case attracted attention from disability rights advocates and educators across the country, because more than 6.4 million children -- or about 13.4 percent of the national public school population -- receive special education under the disabilities act.

"For a long time, special-education kids had no rights," said Charlie Russo, a professor at the University of Dayton's School of Education. "Now critics would say we've gone too far. I see this as coming back to the middle. Special-education students have rights, but they're going to have to trust that school systems are doing what should be done."

Russo said that although the ruling may not reduce the number of parents challenging IEPs, it may reduce the number of these challenges ending up in court.

Montgomery County, the nation's 17th-largest public school system, has more than 17,000 special-education students. It spends a total of $312 million per year on special-education services, including transportation.

The burden of proof is decisive only in the small percentage of cases in which a hearing officer finds that the evidence is essentially tied.

Still, a ruling in favor of the Schaffers could have tipped the balance of power toward parents in such cases nationwide, probably making it more likely that what are now marginal cases would have been filed and won.

With thousands of such disputes litigated each year at a cost to school systems nationwide of $146.5 million in the 1999-2000 school year, the most recent for which data exist, the impact of even a nuanced change in the rules could have been felt both in students' classrooms and in school system finances.

Before yesterday's decision, federal appeals courts had been divided on the issue. In addition to the District's regulation, four states -- Alabama, Alaska, Delaware and Minnesota -- have laws putting the burden of proof on the schools in at least some circumstances.

"I feel very, very sad for parents who feel that their child's needs are not being met but cannot provide for their child's needs on their own," said Jocelyn Schaffer. The Supreme Court's ruling, she said, "just makes it harder for them to be heard."

The Montgomery County superintendent of schools, Jerry D. Weast, hailed the decision as a victory for school systems across the country.

"We defended this case for one simple reason: We didn't want our teachers and staff spending more time in the courtroom than in the classroom," Weast said.

The process for developing an IEP is supposed to be collaborative but can turn combative when parents and the school system disagree on what is best for a child.

O'Connor was joined by Justices John Paul Stevens, Antonin Scalia, Anthony M. Kennedy, David H. Souter and Clarence Thomas. Justices Ruth Bader Ginsburg and Stephen G. Breyer dissented. Chief Justice John G. Roberts Jr. did not vote in the case; it was argued only two days after he took office.

Jocelyn Schaffer sued Montgomery County schools over her son's education; she said the ruling makes it harder for parents to be heard. Jocelyn Schaffer and her husband disagreed with Montgomery County schools' education plan for their learning-disabled son and sued for private-school tuition reimbursement.