MIDDLETON, Idaho – Robin Gilbert didn’t set out to confront gender stereotypes when she split up the boys and girls at her elementary school in rural southwestern Idaho.

But that’s 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 over whether single-sex learning should be continued. Under pressure, single-sex programs have been dropped at schools from Missouri to Louisiana.

“It doesn’t frustrate me,” Gilbert said of the criticism, “but it makes the work harder.”

Single-sex classes began proliferating after the U.S. Education Department relaxed restrictions in 2006. With research showing boys, particularly minority boys, are graduating at lower rates than girls and faring worse on tests, plenty of schools were paying attention.

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 kids of equal educational opportunities. The ACLU claims many schools offer the classes in a way that conflicts with the U.S. 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-authored a review of studies in the journal Science that found research doesn’t support the benefits of single-sex education. Additionally, there are lots of problems whenever you segregate people into groups, Halpern said.

“Stereotyping increases so we really do have lots of data that says it’s just not supported,” she said.

However, proponents have put out their own 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 the struggles boys were having 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 kids in a traditional coed classroom.

For advocates like Dr. Leonard Sax, the founder of the Pennsylvania-based National Association for Single Sex Public Education, the increase in this form of learning is exciting, but it’s 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.

Doug Bonney is legal director of the ACLU of Kansas and Western Missouri, where he successfully challenged single-sex classes in Missouri’s Adrian R-III School District. He argues there’s no proof single-sex classrooms work while there’s plenty of evidence they actually enhance gender stereotypes and lead to sexism.

“This isn’t the right step to address higher dropout rates by boys,” Bonney said. “They promote false stereotypes about sex-based differences that don’t exist. Promoting sex stereotypes can harm both girls and boys.”

Both sides agree the idea is not new and has a long history in private schools. But Galen Sherwin, staff attorney with the ACLU Women’s Rights Project, said its history in public schools is much darker and has roots in the South, where it was broadly instituted in an effort to evade the desegregation requirements of Brown v. Board of Education to try “to prevent black boys from being in the same room as white girls.”

“In the wake of Brown, many schools in the south integrated racially but segregated on the basis of sex,” Sherwin said.

Nancy Levit, a law professor at the University of Missouri-Kansas City, addressed this issue at a meeting of the Association of American Law Schools: “Think about it, in terms of race,” she said. “What would people say if the state paid for an all-white school or an all-black school? As long as there was a racial element nobody would have a problem seeing a constitutional difficulty.”

The analogy drew a heated reaction from Sax, who argues that a federal judge in Kentucky debunked this notion when ruling last year against parents who tried to block single-sex classes at a Breckenridge County school. Critics like the ACLU are out of line when they draw parallels to Brown v. Board of Education, Sax said.

“Either they’re really stupid and not able to grasp what the judge is saying in the ruling, or they’re being deliberately misleading,” he said.