PORTLAND — The ACLU of Maine is demanding that the Sanford School District end its single-gender classes, saying the program appears to violate federal law by relying on harmful gender stereotypes.

Willard School began offering single-gender classrooms as an option for sixth graders three years ago and expanded the program to the fifth grade this year. Superintendent David Theoharides said the district wanted to offer alternatives for students with different learning styles.

Zachary Heiden, legal director for the ACLU of Maine Foundation, said there has to be a very good reason for starting a program that segregates students by categories like gender.

“It has to be a much stronger justification than ‘We think this might be a good idea,'” he said.

In a letter sent to the district today, the organization threatened to file a lawsuit if the single-gender program continues.

Daniel Rose, a lawyer for the district, said he is studying the letter and the allegations.

“The school’s position is that there is no education in Sanford that is based on stereotyping of any kind,” he said.

Based on public records from the district, the ACLU of Maine said it believes improper gender stereotypes are incorporated into the single-gender classrooms. As examples, the organization cited sixth-grade girls discussing current events over cocoa and the boys creating an exercise area in the classroom and earning points toward prizes from the National Football League.

Theoharides said the curriculum is the same for both boys and girls.

The letter sent to the Sanford School District is part of a larger effort by the American Civil Liberties Union. Similar public records requests have been sent to school in Massachusetts, Indiana, Idaho, Washington and Illinois in recent weeks. The organization is reviewing records or has pending requests in other states, including Alabama, Wisconsin, North Carolina, South Carolina and Virginia.