WASHINGTON, May 27 (Reuters) - The U.S. Supreme Court declined on Tuesday to hear a student's challenge on free speech grounds to a Massachusetts public school's decision to bar him from wearing a T-shirt reading "There are only two genders" due to concern about the message's effect on transgender and other pupils.

The justices turned away an appeal by student, who was 12 at the time of the 2023 incident, of a lower court's ruling upholding the ban as a reasonable restriction and rejecting his claim that the school's action violated the U.S. Constitution's protections against government abridgment of speech.

Student Liam Morrison, along with his father and stepmother, sued officials at John T. Nichols Middle School and the town of Middleborough, seeking monetary damages. U.S. District Judge Indira Talwani and then the Boston-based 1st U.S. Circuit Court of Appeals ruled against him.

The 1st Circuit decision stated that "it was reasonable for Middleborough to forecast that a message displayed throughout the school day denying the existence of the gender identities of transgender and gender nonconforming students would have a serious negative impact on those students' ability to concentrate on their classroom work."

Conservative Justice Samuel Alito, in a dissent on Tuesday, called the 1st Circuit ruling flawed.

"As this case makes clear, some lower courts are confused on how to manage the tension between students' rights and schools' obligations," Alito wrote, joined by fellow conservative Justice Clarence Thomas. "Our nation's students, teachers and administrators deserve clarity on this critically important question."

The legal dispute implicates a 1969 Supreme Court precedent in a case called Tinker v. Des Moines Independent Community School District that lets public schools restrict student speech that would "substantially disrupt" a school community.

The issue of transgender rights is front and center in the U.S. culture wars. Since returning to office in January, Republican President Donald Trump has taken a hardline stance on transgender rights, targeting "gender ideology" and declaring that the U.S. government will recognize two sexes: male and female. The Supreme Court on May 6 let Trump's administration implement his ban on transgender people in the military.

While in seventh grade, Morrison wore the T-shirt reading "There are only two genders" to school in March 2023. His lawyers said in court papers he did so to "share his view that gender and sex are identical, and there are only two sexes - male and female."

His lawyers wrote that Morrison "hoped to start a meaningful conversation on gender ideology" as well as to protect other students against ideas that he considered "false and harmful" and to "show them compassionate people can believe that sex is binary."

A teacher reported the shirt to the school principal's office, noting that LGBT students were present that day and expressing concerns that the shirt could disrupt classes. The principal asked the boy if he would be willing to change his shirt and return to class, but he declined. The principal then called the boy's father, who opted to pick up his son from school rather than have him remove his shirt.

School officials cited the dress code in the school's student handbook, which stated: "Clothing must not state, imply or depict hate speech or imagery that target(s) groups based on race, ethnicity, gender, sexual orientation, gender identity, religious affiliation or any other classification."

In May 2023, Morrison again wore the T-shirt to school, but covered the words "only two" with a piece of tape that read "censored," thus bearing the message: "There are (censored) genders." He removed that shirt after being asked by school officials.

During the proceedings, the school system's superintendent said that some students at John T. Nichols Middle School "have attempted to commit suicide or have had suicidal ideations in the past few years, including members of the LGBTQ+ community," and that some of those struggles were "related to their treatment based on their gender identities by other students."

The plaintiffs were represented by the conservative legal group Alliance Defending Freedom. David Cortman, the group's senior counsel and vice president of U.S. litigation, expressed disappointment after Tuesday's decision but said it will "continue to defend the rights of students to speak freely on important issues of the day without government censorship."

"Our legal system is built on the truth that the government cannot silence any speaker just because it disapproves of what they say," Cortman said.

The plaintiffs had sought a court order prohibiting school officials from barring his wearing of the T-shirt and declaring the disputed portions of the dress code unconstitutional.

The Supreme Court is expected to rule by the end of June in a major transgender rights case involving a legal challenge to Tennessee's Republican-backed ban on gender-affirming medical care for transgender minors.