A federal judge on Friday upheld a decision barring a student from wearing a sash honoring her Mexican American heritage to her graduation ceremony after the high school senior sued her Colorado school district.

In the lawsuit, filed on Wednesday in the U.S. District Court for the District of Colorado, lawyers for the student, Naomi Peña Villasano, said she was told by the school principal’s secretary that she could not wear the sash because “allowing that regalia would ‘open too many doors.’”

Lawyers for Ms. Villasano, 18, wrote in the suit that “the sash is a reminder that not all Mexican Americans, including her parents, have the opportunity to graduate from high school and to walk across a graduation stage.”

They added, “By wearing the sash, Naomi represents her family, her identity as a Mexican American and her culture during this important occasion.”

The sash, designed in the style of a serape, was a gift from her older brother and represents the U.S. and Mexican flags. It has the words “Class of 2023” embroidered on it.

In a phone call with Ms. Villasano’s sister-in-law, the principal of Grand Valley High School in Parachute, Colo., about 200 miles west of Denver, reaffirmed that Ms. Villasano would not be permitted to wear her sash at graduation but acknowledged that there was no written school or district policy about regalia worn on or over graduation gowns, the suit said.

Ms. Villasano’s sister-in-law then called the superintendent, who said the district, Garfield County School District 16, did not allow the display of flags because “that would open the door to a student wearing a Confederate flag pin or another flag that would cause offense,” the suit said.

In the hopes of persuading district leaders to change their stance, Ms Villasano this month attended a district board meeting.

“I’m a 200 percenter — 100 percent American and 100 percent Mexican,” she said in her remarks, according to the lawsuit. “I was born in the United States but my parents are Mexican immigrants who came here for a better life.”

Thomas Saenz, president and general counsel of the Mexican American Legal Defense and Educational Fund, which is representing Ms. Villasano, noted that the school said it would allow other students to wear sashes celebrating their Native American or Pacific Islander heritage at graduation. That, he said, violates the Equal Protection Clause of the U.S. Constitution.

“The fact that there are so many cases like this should raise concerns for all of us,” he said.

Ms. Villasano’s lawyers argued that wearing the sash at the graduation ceremony on Saturday was private speech protected by the First Amendment that warranted intervention from the court.

At a hearing in response to an emergency motion seeking permission to wear the sash, Judge Nina Y. Wang sided with the school district.

“Although it is true that many pieces of regalia that complement the cap and gown are worn at the graduate’s option, this court finds that, in the context of Grand Valley High School’s graduation ceremony, any such expression is subject to the school district’s discretion and supervision as a matter of course,” Judge Wang wrote in her ruling.

In a statement, Ms. Villasano said she was at “a loss of words over the decision,” adding that she was “incredibly saddened” that she could not celebrate with her family the way she wanted to.

The district said it was pleased with the decision.

“This is not an issue about a student’s ability to express her pride in her culture and heritage,” Jennifer Baugh, the superintendent, said in a statement. “She, and all her classmates, have an avenue for this expression by decorating their mortar boards on their gradation caps, including appropriate nationalistic endorsements.”

Ms. Villasano’s case comes amid disputes elsewhere about what is protected free speech at commencement ceremonies.

In Oklahoma on Thursday, the State Legislature overrode Gov. Kevin Stitt’s veto of a bill allowing students to wear Native American regalia at high school and college graduations.