PORTLAND — The American Civil Liberties Union of Maine is demanding that the Sanford school district stop offering single-gender classes, arguing that the program appears to be based on gender stereotypes in violation of federal law.

For the past three years, Willard School has offered one all-girl, one all-boy and four co-ed classes at the sixth grade level. The same structure was put in place for the fifth grade this year. The program is voluntary and instruction in areas such as art, music and physical education are co-ed.

Public records provided by the district suggest that gender stereotypes are incorporated into the single-gender classrooms, the ACLU of Maine contends. As an example, the organization points to girls discussing current events over cocoa while the boys have an exercise area and are earning points toward prizes from the National Football League.

The ACLU says programs based on sex stereotypes about how boys and girls learn are not allowed under either the Constitution or Title IX, a federal law that addresses gender equity in federally funded education programs.

The Equal Protection Clause of the Constitution requires a very high threshold for programs that segregate students by categories like gender, said Zachary Heiden, legal director for the ACLU of Maine Foundation.

“It has to be a much stronger justification than ‘We think this might be a good idea,’” and the reason must be clear before implementation, Heiden said.

The organization sent a letter to the school district Monday that raised the possibility of litigation. The action is part of a national ACLU campaign launched Monday called Teach Kids, Not Stereotypes.

Daniel Rose, a lawyer for the district, said he has to review the letter before school officials decide how to respond.

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

Darcy Dube’s daughter, Brenna, was part of the first group of sixth graders who had the gender-specific option. Calm and mild-mannered Brenna thought it would be interesting to see what a class without distracting boys would be like, Dube said.

Brenna had such a good time, Dube said, that her younger brothers, sixth-grader Josiah and fifth-grader Elijah, also opted for single-gender classrooms.

“I think they felt much more comfortable in the learning environment and able to share,” Dube said.

Superintendent David Theoharides said the district wanted to offer alternatives for students with different learning styles. He stressed that boys and girls have the same curriculum.

“The reason we designed this program was certain children had certain needs,” he said. “What I feel in some ways is that the ACLU is saying, ‘You can’t address those needs.’”

The classes are meant to help challenge perceptions such as it’s not cool for boys to read or that girls shouldn’t be interested in math or science, Theoharides said.

The school has had to have a lottery because there have been several more interested students for each grade level than there were available slots, said Principal Chuck Potter.

The ACLU and several education groups said there is no reliable count of single-gender offerings in public schools. The National Association for Single Sex Public Education estimated that there were more than 380 schools that offered them last year, up from about a dozen in 2002.

Single-gender classes have become more common since 2006, when the Bush administration issued regulations meant to promote experimentation, according to Galen Sherwin, staff attorney with the ACLU Women’s Rights Project.

The state Department of Education does not track single-gender programs, which are a local decision, said David Connerty-Marin, a department spokesman.

The National Association for Single-Sex Public Education maintains that girls and boys learn in different ways and that single-sex education can break down gender stereotypes when implemented well. Board members of the American Council for CoEducational Schooling, however, criticizes claims about the differences in boys’ and girls’ brains as “pseudoscience” and argues that the single-sex education in public schools constitutes institutional sexism.

The teacher for the sixth-grade girls’ class and the unusual nature of a gender-specfic classroom were part of the draw for Debbie Little. Her elder daughters — Tiphanie, who is now in the seventh grade and Kayleigh, who is in sixth — were enthusiastic about the prospect, she said, and it will be very disappointing if the youngest, Ariannah, does not have the same option.

“I just felt their learning environment — learning experience — in the all-girl class was a positive experience for them,” she said.