ASHINGTON, March 29 - The Supreme Court ruled on Tuesday that the federal law barring sex discrimination in schools and colleges also prohibits school officials from retaliating against those who bring sex discrimination complaints.

The 5-to-4 ruling resolved conflicting interpretations in the lower courts over the scope of the law, Title IX of the Education Amendments of 1972.

While the margin was narrow, the language of Justice Sandra Day O'Connor's majority opinion was sweeping. For Title IX's advocates, who have been placed on the defensive in recent years by complaints from critics that the law's obligations are too burdensome, the ruling was a decisive victory.

Most significantly, the court held that the law's protections extended beyond those who are themselves the victims of sex discrimination in the usual sense, applying as well to third parties who complain about sex discrimination on behalf of others.

The plaintiff in this case was a male gym teacher, Roderick Jackson, who lost his position as a girls' basketball coach at a high school in Birmingham, Ala., after he complained that the girls' team had to play and practice under inferior conditions compared with the boys' team.

"Retaliation against a person because that person has complained of sex discrimination is another form of intentional sex discrimination," Justice O'Connor said. She added, "The statute is broadly worded: it does not require that the victim of the retaliation must also be the victim of the discrimination that is the subject of the original complaint."

The decision, Jackson v. Birmingham Board of Education, No. 02-1672, gives Mr. Jackson the opportunity to go back to Federal District Court in Birmingham and try to prove that his dismissal as a coach was, in fact, retaliation for his complaints. He has remained at the high school as a gym teacher, but his removal as coach in 2001 cost him extra pay and credits toward retirement.

Because the district court dismissed his lawsuit in a decision affirmed by the United States Court of Appeals for the 11th Circuit, in Atlanta, Mr. Jackson has not yet had a chance to make his case. He is seeking compensatory damages as well as an injunction requiring the Birmingham schools to comply with Title IX.

Marcia D. Greenberger, co-president of the National Women's Law Center, which represented Mr. Jackson, called the decision "a slam-dunk for everyone who cares about equal opportunity." The Bush administration had entered the case on Mr. Jackson's behalf, defending a federal regulation adopted shortly after Title IX became law that interpreted the statute as protecting against retaliation.

While Justice O'Connor took account of the federal regulation in her opinion, she said there was no need to rely on it because "the statute itself contains the necessary prohibition" against retaliation.

Title IX was written as a condition on the receipt of federal money; by their acceptance of federal money, schools and colleges agree not to discriminate "on the basis of sex." Unlike Title VII of the 1964 Civil Rights Act, which explicitly prohibits retaliation against those who complain about discrimination on the job, Title IX does not actually mention retaliation.

That silence led Justice Clarence Thomas, in a dissenting opinion, to describe the outcome of the case as contrary to the language of the statute. Because the law does not refer to retaliation, he said, retaliation could be prohibited conduct only if it came within the law's definition of discrimination "on the basis of sex."

But that could not be, Justice Thomas continued, "because retaliatory conduct is not discrimination on the basis of sex." He said that "Jackson does not claim that his own sex played any role, let alone a decisive or predominant one, in the decision to relieve him of his position." Rather, Justice Thomas said, Mr. Jackson's claim was "founded on the attenuated connection between the supposed adverse treatment and the sex of others."

Addressing that objection, Justice O'Connor said that "because Congress did not list any specific discriminatory practices when it wrote Title IX, its failure to mention one such practice does not tell us anything about whether it intended that practice to be covered." She said that looking at the context and structure of the statute, it was clear that "where the retaliation occurs because the complainant speaks out about sex discrimination, the 'on the basis of sex' requirement is satisfied."

The majority opinion was joined by Justices John Paul Stevens, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer. Chief Justice William H. Rehnquist joined the dissent, as did Justices Antonin Scalia and Anthony M. Kennedy.

When the case was argued in November, Walter Dellinger, representing Mr. Jackson on behalf of the National Women's Law Center, emphasized that it was important to protect third parties like coaches and teachers from retaliation for reporting sex discrimination because these adults are in a better position than school children to detect problems and bring their complaints forward.

Justice O'Connor endorsed this argument in a wide-ranging discussion of the policy behind the statute. "Teachers and coaches such as Jackson are often in the best position to vindicate the rights of their students," she said. Addressing the retaliation issue generally, she said: "Reporting incidents of discrimination is integral to Title IX enforcement and would be discouraged if retaliation against those who report went unpunished." She added that "without protection from retaliation, individuals who witness discrimination would likely not report it, indifference claims would be short-circuited, and the underlying discrimination would go unremedied."

In a second decision on Tuesday, the court ruled in favor of a small upstate New York city, Sherrill, in a property tax dispute with the Oneida Indian Nation.

In the late 1990's, the Oneidas purchased property in Sherrill that had once been contained within the tribe's original reservation but had not been owned by the tribe since 1805. The tribe then refused to pay property taxes on the ground that the parcels of land should now be treated as "Indian country" and immune from tax liability. To the alarm of a number of upstate communities, the federal district court and the United States Court of Appeals for the Second Circuit upheld the Indians' argument.

The Supreme Court overturned the Second Circuit's ruling in an 8-to-1 opinion by Justice Ginsburg. She said that "in light of the long history of state sovereign control over the territory" and the Oneidas' long delay in reacquiring the properties, the tribe should not be allowed to pursue such a "disruptive" remedy for long-ago wrongs. Justice Stevens dissented from the opinion, City of Sherrill v. Oneida Indian Nation, No. 03-855.