A U.S. federal appeals court ruled on Tuesday in favour of a transgender student who sued her school for discrimination after she was barred from using the boys' restroom.

Transgender student David Grimm, 17, born female but who identifies herself as male, won the case at the U.S. Fourth Circuit Court of Appeals, which sent the case back to a lower court.

Grimm sued Gloucester County School Board for sex discrimination under the 1972 Title IX Act. A lower court threw out her case, but she appealed.

She said she started using the boys' restroom in December 2014 after being granted permission by the school principal.

"I'm not a girl. I'm not using the girl's restroom," Grimm said, according to the Christian News Network.

She was first allowed to use the nurse's room but it was a three-minute walk from class. She then asked the principal to allow her to use the boys' restrooom.

The school district drew up a policy that students must use restrooms according to their biological sex or else use a private bathroom.

Grimm filed the suit with the help of the American Civil Liberties Union (ACLU).

"By excluding Gavin—a transgender boy—from the boys' restrooms because the school board does not deem him to be 'biologically' male, the school board, under color of state law, has treated and continues to treat Gavin differently from similarly situated students based on his gender," the suit read.

District Court Judge Robert Doumar ruled against Grimm last September, saying "Title IX prohibits discrimination on the basis of sex and not on the basis of other concepts such as gender, gender identity, or sexual orientation."

ACLU appealed the case.

"At the heart of this appeal is whether Title IX requires schools to provide transgender students access to restrooms congruent with their gender identity," the appeals court ruled. "We conclude that the department's interpretation of its own regulation . . . as it relates to restroom access by transgender individuals, is . . . to be accorded controlling weight in this case."

The lower court will rehear Grimm's case.

The Alliance Defending Freedom (ADF) opposed the ruling. "Protecting students' privacy while using the restroom, showers, or locker rooms and not forcing them to share intimate settings with members of the opposite sex is not only legal, it's an important duty of officials who watch over our children," it said.