MIDDLETON, Idaho — Robin Gilbert did not set out to confront gender stereotypes when she split up the boys and girls at her elementary school in rural southwestern Idaho.
But that is exactly what happened, with her Middleton Heights Elementary now among dozens of public schools nationwide being targeted by the American Civil Liberties Union in a bitter struggle about whether single-sex learning should be continued. Under pressure, single-sex programs have been dropped at some schools.
‘‘It doesn’t frustrate me,’’ Gilbert said of the criticism, ‘‘but it makes the work harder.’’
While Gilbert’s school is thought to be the only one in Idaho offering single-sex classes, the movement is widespread in states like South Carolina, which has more than 100 schools that offer some form of a single-gender program.
Single-sex classes began proliferating after the US Education Department relaxed restrictions in 2006. With data showing boys, particularly minority boys, are graduating at lower rates than girls and faring worse on tests, plenty of schools were paying attention.
In 2002, only about a dozen schools were separating the sexes, according to the National Association for Single Sex Public Education, an advocacy group. Now, an estimated 500 public schools across the country offer some all-boy and all-girl classrooms.
Proponents argue the separation allows for a tailored instruction and cuts down on gender-driven distractions among boys and girls, such as flirting. But critics decry the movement as promoting harmful gender stereotypes and depriving children of equal educational opportunities.
The ACLU says many schools offer the classes in a way that conflicts with the Constitution and Title IX, a federal law banning sex discrimination in education.
Researchers also have weighed in. Diane F. Halpern, a former president of the American Psychological Association, co-wrote a review of studies last fall in the journal Science that found research does not support the benefits of single-sex education.
‘‘Stereotyping increases so we really do have lots of data that says it’s just not supported,’’ she said.
But proponents have conducted studies, showing the benefits of separating students. Middleton Heights Elementary cited the research when it first piloted single-sex classes in a few grades. The goal was to address struggles boys had in reading.
The idea proved so popular that single-sex classes have expanded throughout the school. Parents can opt out, a choice required by law, if they want their children in traditional mixed classrooms.
In the single-sex classes, teachers use microphones that allow them to electronically adjust the tone of their voice to match the level that research suggests is best for boys. When preparing for a test, the boys may go for a run, or engage in some other activity, while girls are more likely to do calming exercises, such as yoga.
On a recent tour, Gilbert peeked into a classroom of third-grade boys, who had decorated their walls with a camping theme, with construction paper campfires and a sign that read ‘‘fishing for books.’’
Next door, third-grade girls opted for an ‘‘under the sea’’ motif.
They are taught the same curriculum. They still eat lunch and play at recess together, but the differences in their learning environments are apparent, from blue chalkboards in the boys’ classrooms, to the red paper hearts that decorated one of the girls’ classrooms.
These environments are driven by student interests and what they are learning at the time, Gilbert said.
Dr. Leonard Sax, the founder of the Pennsylvania-based National Association for Single Sex Public Education, said the movement is about breaking down gender stereotypes, not promoting them.
‘‘We want more girls engaged in robotics and computer programming and physics and engineering,’’ Sax said. ‘‘We want more boys engaged in poetry and creative writing and Spanish language.’’
For advocates such as Sax, the increase in this form of learning is exciting, but it is troubling for others.
The ACLU launched a national campaign, Teach Kids, Not Stereotypes, in May and sent cease-and-desist letters to school districts in Maine, West Virginia, Alabama, Mississippi, and Virginia.
The group also asked state officials to investigate single-sex programs in Florida, while sending public record requests to schools in another five states, including to Gilbert’s school in Idaho.