PORTLAND — The Maine Supreme Judicial Court on Thursday reversed a lower court’s decision that banned a transgender child from using the girls’ bathroom in Orono schools.

In a 5-1 decision, the justices said that Superior Court Justice William Anderson erred when he ruled in favor of what is now Riverside RSU 26.

“Our opinion must not be read to require schools to permit students casual access to any bathroom of their choice,” Justice Warren Silver wrote for the majority. “Decisions about how to address students’ legitimate gender identity issues are not to be taken lightly. Where, as here, it has been clearly established that a student’s psychological well-being and educational success depend upon being permitted to use the communal bathroom consistent with her gender identity, denying access to the appropriate bathroom constitutes sexual orientation discrimination in violation of the [Maine Human Rights Commission].”

The incident that sparked the court case began in 2007 when a child, who was born male but identifies as female, was forced to stop using the girls bathroom at the Asa Adams Elementary School in Orono. She was told to use a staff bathroom after the grandfather of a male student complained.

Gay & Lesbian Advocates & Defenders of Boston represented the girl and her family.

“This is a momentous decision that marks a huge breakthrough for transgender young people,” said Jennifer Levi, director of GLAD’s Transgender Rights Project, who argued the case before the justices last year. “Schools have a responsibility to create a learning environment that meets and balances the needs of all kids and allows every student to succeed. For transgender students, this includes access to all school facilities, programs and extracurricular activities in a way that is consistent with their gender identity.”

“A transgender girl is a girl and must be treated as such in all respects, including using the girls’ restroom. This ruling is consistent with what educators and human rights commissions – including the Maine Human Rights Commission — around the country have concluded,” said GLAD Senior Attorney Bennett Klein, who was co-counsel with Levi in the case.

Portland attorney Melissa Hewey, who represents the school district, also praised the decision.

“In its decision issued today, the Maine Supreme Court confirmed what has been the Orono School Department’s contention all along — that its personnel acted with ‘tremendous sensitivity and insight’ and undertook a ‘rational and compassionate approach’ while ‘working in uncharted territory,’” Hewey said Thursday. “And this, from our perspective, is the most important part of the decision.

“The court has also provided helpful guidance about how to handle this issue that is becoming more and more common in schools around the state and the country,” she continued. “Now that its obligations have been clarified, the Orono School Department will take all necessary steps to ensure that it complies with the law.”

The girl’s parents, using the names John and Jane Doe and Susan Doe for their daughter, and the MHRC sued the Orono school district, now called Riverside RSU 26, and then Superintendent Kelly Clenchy after the commission ruled in the girl’s favor.

Since the lawsuit was filed the family has publicly identified themselves. The parents are Wayne and Kelly Maines. Their daughter at the center of the case, now 15, is Nicole Maines. Her twin brother is Jonas Maines.

The justices’ questions at oral arguments in the case in June in Bangor focused on a conflict between a law passed in the 1920s that requires separate bathrooms for boys and girls in schools and the provision enacted in 2005 in the Maine Human Rights Act that prohibits discrimination on the basis of sexual orientation.

Chief Justice Leigh I. Saufley and Justices Donald Alexander, Ellen Gorman and Joseph Jabar joined Warren in the majority. Justice Andrew Mead dissented. Justice Jon Levy recused himself from the case for reasons that have not been made public.