WASHINGTON — After two decades of giving school officials wide leeway to search students for drugs or weapons, the Supreme Court set a legal limit Thursday, ruling out of bounds the strip search of a 13-year-old girl who was suspected of hiding pain relief pills.

In an 8-1 decision, the court called this search degrading, unreasonable and unconstitutional.

Justice David H. Souter, in what could be his final opinion before his retirement, said a strip search is “categorically distinct” from other efforts to find drugs or weapons on campus because it is embarrassing and humiliating to the children who are targeted.

In the past, the court has said school officials can search purses, backpacks or lockers if they have reason to believe a student has drugs. And twice, they have upheld mandatory drug testing of high schoolers, including athletes, even when there was no reason to think any of them was using drugs.

But requiring a student to remove her clothes goes too far, Souter said. He suggested such as search would be justified only if a school official had strong reason to believe a student was hiding a dangerous drug or a weapon in his or her underwear.

Savana Redding, the Arizona woman whose suit led to Thursday’s ruling, said she was pleased and surprised by the outcome.

“I’m very excited and very happy knowing it means this is not likely to happen to anyone else at school,” she said. Now 19, Redding said she will be taking classes this fall at Eastern Arizona College.

Her lawyer, Adam Wolf of the American Civil Liberties Union, said he was encouraged that the court had made clear that students have privacy rights at school.

“School children are not little prisoners subject to search. This says the Constitution applies in school, and children have rights that must be respected,” he said.

School lawyers read the decision as nearly prohibiting strip searches.

“We don’t think it is a horrible decision, but it is going to limit the discretion of school officials. They will think long and hard before they authorize a strip search in the future,” said Naomi Gittins, a lawyer for the National School Boards Association.

In 2003, Redding was an eighth grader in the small town of Safford, Arizona, near the border with New Mexico. That fall, one boy had gotten violently ill from taking pills at school. When another girl was found with several white pills hidden in a folder, she told vice principal Kerry Wilson she got them from Savana Redding. The pills were prescription-strength ibuprofen pills, equivalent to two Advil tablets.

When asked, Savana said she knew nothing of the pills. Her backpack was searched. When no pills were found, Wilson sent her to a nurse’s office, where she was told to remove her outer clothes and to pull out her bra and underwear to check for hidden pills.

Nothing was found, and the school officials did not apologize when Savana’s mother April confronted them over the strip search of her daughter. The Reddings then filed suit, alleging a violation of her rights under the Fourth Amendment, which forbids “unreasonable searches” by the government.

The decision in Safford Unified School District v. Redding was only a partial victory, however. The justices threw out their suit against Wilson and other school officials on the grounds that the law against strip searches was not “clearly established” in 2003 at the time of incident.

Under federal law, public officials can be sued and held liable if they violate a person’s “clearly established” rights under the Constitution. Souter admitted that until Thursday, judges around the nation were divided over whether a strip search at school was unconstitutional.

Thursday’s decision sets a standard for all future school searches, but it may result in no compensation for the Savana and her mother. The court sent the case back to Arizona to consider whether the school district itself may face some liability.

Souter agreed the vice principal had reasonable grounds for questioning Savana about drugs and for searching her backpack. But he went way too far, he added.

“In sum, what was missing,” he said, “was an indication of danger to the students from the power of the drugs or their quantity, and any reason to suppose that Savana was carrying pills in her underwear. We think that combination of these deficiencies was fatal to finding the search reasonable,” he wrote.

Only Justice Clarence Thomas dissented. He complained the ruling “grants judges sweeping authority to second-guess the measures that these officials take to maintain discipline in their schools and ensure the health and safety of the students in their charge.”

It is the second time this week Thomas dissented alone in a major case. On Monday, the court rejected a challenge to the Voting Rights Act, but Thomas said he would have struck it down as unconstitutional.

In the school case, Justices John Paul Stevens and Ruth Bader Ginsburg said they would have gone further and upheld a liability ruling against the school officials in this case. “I have long believed that it does not require a constitutional scholar to conclude that a nude search of a 13-year old child is an invasion of constitutional rights of some magnitude,” Stevens wrote.

Ginsburg, the court’s only woman, said the vice principal’s conduct in this case was inexcusable. He had no real evidence to suspect Savana of wrongdoing. He did not contact her mother, and he made her sit in the office for several hours after the strip search, she said.

“An abuse of authority of that order should not be shielded by official immunity,” Ginsburg wrote.

While Souter will retire after Monday when the final rulings are handed down, his designated successor – Judge Sonia Sotomayor – has taken a similarly strong stand against strip searches. In 2004, she voted to uphold a suit against several Connecticut officials who authorized the strip search of two young girls at a juvenile detention center.

These “were troubled adolescent girls facing no criminal charges,” Sotomayor wrote. Because of their potential to humiliate the victims, “we should be especially wary of strip searches of children,” she said.

Seven states have laws that forbid strip searches at school. They are California, Iowa, New Jersey, Oklahoma, South Carolina, Washington and Wisconsin.