Two years ago, Alexis Lightcap, then a student at Boyertown Area Senior High School in Pennsylvania, stumbled into a national debate when she walked into the girls’ bathroom and saw a man. Shocked, Lightcap ran out. “We actually have video footage of her fleeing the restroom in fear,” says Christiana Holcomb, legal counsel for Alliance Defending Freedom (ADF), an advocacy group that defends religious freedom, in a video about school bathroom policies.

Another student at Boyertown had a similar experience in the boys’ locker room. A girl who identified as a boy “was standing in her sports bra, [by] her locker,” says Holcomb. The first student “was standing there in his underwear.” When this student and Lightcap reported the incidents to the school’s principal, he dismissed their concerns. They were “told they needed to tolerate it and make it as natural as possible,” Holcomb says, because this was now the school’s policy.

This was the first time Lightcap and other students had heard of any such policy. “I found out about the [new school] policy after I encountered a male in the bathroom,” Lightcap says. The school enacted the new policy—without notifying parents or students—allowing students to use the bathroom that aligned with their self-defined gender “identity” rather than their biological sex. With help from ADF, Lightcap and other students filed a lawsuit, Doe v. Boyertown Area School District, arguing that the new policy violated the privacy rights of non-transgender students. In July, the Court of Appeals for the Third Circuit rejected their claim. But last week, Lightcap and her fellow plaintiffs petitioned the Supreme Court to hear the case.

At the heart of this case—as well as several transgender bathroom cases that have worked their way through the courts in recent years—lies the interpretation of Title VII and Title IX. These famous laws (the first from the Civil Rights Act of 1964, the second from the Education Amendments of 1972) prohibit employers and institutions that receive federal funds from discriminating on the basis of several qualities, including sex.

“For almost 20 years, courts have held that Title VII and Title IX protect trans people,” says Josh Block, senior staff attorney at the American Civil Liberties Union (ACLU). Block represented Gavin Grimm, a transgender student who in May won a case against a Virginia school board that prohibited Grimm from using the boys’ bathroom. Last year, the Supreme Court remanded Grimm’s case, since the Trump administration had just reversed an Obama-era notice for public schools to accept transgender students’ preferences. But now, under the new policy and with a conservative majority on the bench, the Boyertown appeal has a chance of bringing the debate back to the High Court.

When it comes to preventing discrimination on the basis of sex, says Block, “The law needs to be interpreted according to its plain language.” But the language at issue—namely, what the word “sex” means—is not at all plainly understood. Both Title VII and Title IX were passed long before some activists began arguing that “sex” is not limited to the biological distinction between male and female but includes “gender identity.” Given the continuing controversy over the definition of sex in a transgender age, how should the courts navigate conflicts like the one that emerged in Boyertown?

In the past decade, according to the Williams Institute at the University of California, Los Angeles, and the National Institutes of Health, the number of people who identify as transgender in the United States has risen to over one million. One clinic in the U.K. that specializes in treating transgender patients saw its child referrals quadruple between 2012 and 2017, and the approximately 40 “gender health facilities” in this country also report a significant rise in referrals.

The ACLU’s Block thinks acceptance of transgenderism will follow a similar path to acceptance as the one the culture made with regard to gay rights. “As was the case with discrimination against lesbian, gay, and bisexual people,” he says, “as people came to see more [of them] as people, as they came to understand them as their coworkers, as their friends . . . a lot of stereotypes and assumptions faded away.”

But even if cultural norms change, extending Title IX to include transgender rights is far from a settled legal question. ADF’s Holcomb notes, “There are only a couple of circuit-level decisions [that have granted] preliminary injunction. We’re going to have to press forward, and the Supreme Court will ultimately have to resolve the issue,” she says. “The definition of sex by Congress has always been clear, undisputed,” she continues. “[It’s] always been basic biological differences.”

In their petition to the Supreme Court, the Boyertown plaintiffs have invoked the same logic. “The Third Circuit redefined ‘sex’ in the privacy and Title IX contexts as depending solely on a student’s subjective perceptions and feelings,” they argue. “Nothing in this Court’s precedents or the plain language of Title IX supports such a redefinition.”

Lightcap says cases such as hers aren’t about transphobia or denying others their rights—they are about protecting the privacy and safety of all students. “I don’t have a problem sharing a bathroom with someone who identifies as transgender—provided they are the same sex I am,” Lightcap wrote in an opinion piece. “I do have trouble with a policy that says anyone who’s in an opposite-sex mood today can stroll in and observe me in my intimate moments.” The expansive bathroom policies of schools like Boyertown make it impossible to prevent that from happening.

Lightcap emphasizes just how young the students affected by these policies are. “There are 13-, 14-year-old girls in this school,” Lightcap says. “I don’t want them walking into a male in the bathroom.”

“These are minors,” Holcomb says. “However we might disagree more broadly, culturally, and perhaps in the college setting, that’s one thing,” she adds. “[But here] we’re really talking about children.” For her part, Lightcap remains optimistic. “I’m going wherever God leads me, and this is where he’s led me,” she says. “I’m going to continue fighting.”