Becky Pepper-Jackson was 11 years old and a mediocre runner when she sued the State of West Virginia over a law that barred transgender girls like her from joining girls’ sports teams.

For the next four years, Ms. Pepper-Jackson’s status on the school track team was in doubt as judges blocked the law, then unblocked it and then blocked it again.

Now 15, she is an accomplished shot-putter and discus thrower. A decision about whether she can compete for her final two years of high school will now fall to the Supreme Court, which on Tuesday will hear her challenge to the West Virginia law that supporters said was intended to ensure fair competition for female athletes.

In an interview, Ms. Pepper-Jackson said she was devastated when the law took effect.

“I want to be able to play on the team with my friends, and that’s all I’ve wanted to do,” she said.

Ms. Pepper-Jackson is the only known athlete in West Virginia who would be subject to the law.

But her case — and a separate matter from Idaho dealing with college sports that the court is also hearing on Tuesday — has implications far beyond her personal sports saga.

Twenty-five other states have enacted similar restrictions since 2020, amid a backlash to transgender students participating in school sports and transgender rights broadly. However the Supreme Court rules this year, the decision could affect all of those states, and the student athletes who seek to play in schools and colleges there.

The outcome may also further influence the national conversation about transgender people in American society. President Trump targeted the community in his last presidential campaign and declared at his inauguration that it would be the “official policy of the United States government that there are only two genders: male and female.”

In February, surrounded by young female athletes in uniform, Mr. Trump signed an executive order directing agencies to withdraw federal funding from schools that allow transgender athletes in girls’ and women’s sports. The next day, the N.C.A.A., which for years had tried to balance fairness with inclusion in college sports, announced it would bar trans women from competing in women’s sports.

The Supreme Court has similarly moved against transgender rights and seems likely to reject the trans athletes’ claims based on recent rulings. After protecting transgender people from workplace discrimination in a 2020 decision written by Justice Neil M. Gorsuch, a Trump appointee, the court in June upheld a Tennessee law that prohibits certain medical treatments for transgender adolescents. In a case they heard in October, the justices also seemed poised to strike down a Colorado law that bars mental health professionals from seeking to change a minor’s gender identity through so-called conversion therapy.

Lawyers for West Virginia and Idaho, the Trump administration and other supporters say the sports laws are needed, claiming transgender girls and women have a competitive edge. Allowing them to compete on girls’ teams, they say, takes away opportunities for female athletes and reverses five decades of progress for women in sports since the passage of Title IX, the civil rights law that bars sex discrimination in schools.

West Virginia’s attorney general, John McCuskey, whose office is defending the law, said the restrictions like the one his state adopted in 2021 were not only constitutional but also “common sense” because men are “bigger, faster and stronger.”

In an interview, Mr. McCuskey, a father of two daughters who play soccer, said, “When you watch your children putting all of this insane and incredible internal effort into being great at something, the idea that their place on one of those teams would be taken by somebody that has a noticeable and obvious unfair advantage does make it personal to me.”

Efforts to bar transgender athletes from women’s sports have seen broad support in polls. A New York Times/Ipsos poll conducted last year found that nearly 80 percent of Americans, including two-thirds of Democrats, opposed allowing transgender women to compete in women’s sports.

The case has attracted the attention of former Olympians, soccer standouts and N.C.A.A. stars, who submitted competing briefs. Some of the athletes urged the court to ensure “a level playing field” for female athletes by limiting participation to people assigned female at birth. Others said the justices should not prevent trans girls and women from experiencing the benefits of being part of a team.

The Idaho law was challenged by Lindsay Hecox, a college student who tried out for the women’s track and cross-country teams and played club soccer. Challenges to both states’ laws argue that the restrictions violate the Constitution’s guarantee of equal protection, which requires the government to have valid reasons for treating people differently. Two appeals courts have sided with the students and blocked enforcement of the laws.

A wrinkle in the Idaho case came when Ms. Hecox, now 24 and a senior at Boise State University, moved to withdraw her suit in September, saying she would refrain from playing school-sponsored women’s sports. Ms. Hecox, who receives testosterone suppression and estrogen treatment, cited negative public scrutiny that would distract from her academics and her plans to graduate in May. In response, the justices said they would defer a decision about whether to dismiss her case as moot until after Tuesday’s argument.

In West Virginia, Ms. Pepper-Jackson, who was assigned male at birth, transitioned in third grade. She has legally changed her name, and her birth certificate recognizes her as female. She says that because she takes puberty-blocking medication, she has not gone through typical male puberty and does not have the muscle mass, bone density or hormone levels of boys.

In an interview, Ms. Pepper-Jackson said she was inspired by her two older brothers to join the middle school cross-country team and was crushed when West Virginia’s governor signed the law a few months before tryouts.

With the help of the American Civil Liberties Union and Lambda Legal, she and her mother, Heather Jackson, filed a lawsuit, saying the ban violated Title IX protections against sex discrimination in schools and the 14th Amendment’s equal protection guarantee. Among the questions for the justices on Tuesday is whether the word “sex” under Title IX means only biological sex or includes gender identity.

Ms. Pepper-Jackson’s improvement on the field has made her legal case more challenging, as opponents have argued she has taken opportunities from other girls.

After a lackluster cross-country performance in sixth grade in which, according to her court filings, she regularly finished “near the back of the pack,” Ms. Pepper-Jackson said her coach recommended that she instead try throwing the shot put and discus. Her mother created a practice pit in the backyard, mapping out the precise measurements in the space where an aboveground pool once stood.

Ms. Pepper-Jackson said she stayed late for double practice sessions at school and lifted weights all summer. She carried a broomstick behind her neck to work on keeping her arms up and back straight.

As a freshman at Bridgeport High School, she qualified for the state tournament last May, and placed eighth in shot put and third in discus. West Virginia’s lawyers, citing state athletic statistics, said in just one season, she beat out and “displaced nearly 400 female competitors.”

In response, Ms. Pepper-Jackson said she recognized that there were a limited number of spots for athletes in the championships, but she put in the work to earn one.

“I don’t think they’re understanding how much effort I am truly putting in behind the scenes,” she said in a Zoom interview, with her mother and two rambunctious Great Danes in the background. “I don’t think they care.”

A teammate who initially beat Ms. Pepper-Jackson in throwing competitions later objected to her participation after Ms. Pepper-Jackson outperformed her.

In a filing at the Supreme Court, Adaleia Cross, 17, said she was angry and discouraged that Ms. Pepper-Jackson had “taken her spot” in a 2023 competition.

Ms. Cross said she was initially reluctant to speak out for fear that she would be labeled transphobic, but decided to do so in part because she alleged that Ms. Pepper-Jackson had made vulgar remarks in the middle school locker room, making her feel embarrassed and fearful.

The Supreme Court, like all appellate courts, is not supposed to consider facts and allegations, like Ms. Cross’s, that were not part of the record in the lower courts.

Ms. Cross and her family declined to be interviewed but have shared their stories on a website maintained by her lawyers. They are represented by the Alliance Defending Freedom, the Christian conservative legal organization that is also assisting the attorneys general from Idaho and West Virginia.

The middle school investigated Ms. Cross’s complaint in 2023 and concluded that the allegations “could not be substantiated,” according to a letter from the school system’s lawyer reviewed by The New York Times.

Ms. Pepper-Jackson denied the allegations as well.

“I was not raised like that,” she said.

Ms. Pepper-Jackson’s case first reached the Supreme Court on an emergency basis in 2023. The justices at that time issued a brief order allowing her to compete while litigation continued. The order gave no reasons. Justices Clarence Thomas and Samuel A. Alito Jr. dissented.

In April, a divided panel of the U.S. Court of Appeals for the Fourth Circuit said the law could not be enforced against Ms. Pepper-Jackson.

The majority found that the law discriminated in violation of Title IX by preventing trans girls — but not trans boys — from participating on teams consistent with their gender identity and did so “regardless of whether any given girl possesses any inherent athletic advantages based on being transgender.”

Given that Ms. Pepper-Jackson had not gone through male puberty, allowing her to play on a boys’ team would require her to share the field with boys who are “larger, stronger and faster than her because of the elevated levels of circulating testosterone she lacks,” wrote Judge Toby J. Heytens, joined by Judge Pamela A. Harris.

In dissent, Judge G. Steven Agee said Ms. Pepper-Jackson’s participation had taken opportunities from girls who were born female, precisely what West Virginia’s law was intended to prevent.

“Allowing transgender girls — regardless of any advantage — as participants in biological girls’ sports turns Title IX on its head and reverses the monumental work Title IX has done to promote girls’ sports from its inception,” he wrote.

Throughout the litigation, West Virginia’s lawyers have said Ms. Pepper-Jackson could instead compete on the boys’ track and field team. But she insists that is not an option. Her reasoning: “I’m not a man.”