They learn the same curriculum, they still lunch and play at recess together, but the differences in their learning environments are apparent, from the blue chalkboards in the boy classrooms, to the red paper hearts that decorated the wall of one of the girl's classrooms.
These environments are driven by student interests and what they're learning at the time, Gilbert said.
Dr. Leonard Sax, the founder of the Pennsylvania-based National Association for Single Sex Public Education, contends 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 in a telephone interview. "We want more boys engaged in poetry and creative writing and Spanish language."
For advocates like Sax, 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. 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.
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.