Girls and boys mingled as they jumped off the bus, greeted teachers and headed into Battle Creek Middle School one morning last week. But within minutes, they split up: Boys headed to the auditorium for a back-to-school meeting, while girls went to the gym.

They spend most of their school days apart, in separate classes that teachers say are tailored to their differing needs. Battle Creek, in St. Paul, is among a growing number of public schools nationwide venturing into single-sex education, a trend that's fueling both experimentation and debate.

"What single-gender education allows us to do is really focus on the needs of girls and boys, and create a climate where kids feel comfortable," said Battle Creek principal Jocelyn Sims.

It's unclear how many single-sex public schools exist, but estimates from several advocates and opponents fall between 80 and 200, up from a handful 15 years ago. Hundreds more co-ed schools now offer at least one single-sex class, and several experts agree that the number has risen since 2006, when the U.S. Department of Education issued new rules making it easier for districts to launch such schools and classes.

This fall, Rosemount, Hastings and Henry Sibley high schools are among those offering new single-sex alternatives to individual classes such as strength training or algebra. A handful of Minnesota public schools divide at least some boys and girls for most of the school day, including Lucy Craft Laney School and Best Academy in Minneapolis.

But even as the schools and classes spread, research is inconclusive on whether they help. A 2008 report prepared for the U.S. Department of Education, which included a review of studies on single-sex schools, said the results were mixed, though they suggested some support for the idea.

Sims admits that it's hard to prove that single-sex classes have caused the "incremental growth" the school has seen on tests. "I cannot say that single-gender schooling makes or breaks the learning," she said.

Many advocates argue that single-sex schooling can help build a strong academic culture, reduce social distractions or bust stereotypes such as the idea that poetry is girly or computer science is for boys. But critics counter that it's unproven and often results in the promotion -- not reduction -- of harmful gender stereotypes.

Sexual discrimination?

Opponents argue that the 2006 rules undercut Title IX, a law barring sex discrimination in federally funded schools. Some particularly object when schools justify separating boys and girls by citing consultants who say that brain differences between the sexes tend to result in distinct learning needs.

"There's no research that shows that any of those [sex-based] biological differences should lead to differences in teaching or require separation of boys from girls," said Galen Sherwin, a staff attorney at the Women's Rights Project of the American Civil Liberties Union. All students learn differently because of many factors that don't break down simply along gender lines, the ACLU argues.

Administrators at many schools cite author and school consultant Leonard Sax as a resource for information about single-sex education.

In a 2005 book, "Why Gender Matters," Sax wrote that there are "consistent and significant brain-based sex differences in how girls and boys learn geometry and how they understand literature."

Among other things, Sax wrote that many young boys are "energized by confrontation" while "few young girls will flourish in high-pressure" situations. A teacher working with a girl should smile and look her in the eye, but sit shoulder-to-shoulder with a boy and refrain from smiling.

Sax's critics say those claims amount to stereotyping. He now says that the book contains "major errors" related to some research he cited, but still argues that co-ed classes are often taught in a way that shortchanges one gender -- girls in physics, for example. He said he supports single-sex education because it can help "challenge the sexist culture in which we live."

To Battle Creek teacher Stephanie Drow, one key benefit of separating boys and girls is that "they act more age-appropriate." Girls in her classes are more relaxed, she said, while in the co-ed classes she used to teach, "It was always about who's trying to get a boy."

Several boys and girls agreed. Without students of the opposite sex in the room, "We can just act like ourselves," said eighth-grader Chimua Lor.

Teachers also said they notice gender differences in their classes and tweak lessons accordingly. For example, boys tend to be more competitive and crave more physical activity, they said, while girls are more organized.

Battle Creek officials began splitting up some classes by gender in 2005 under a former principal who hoped to boost enrollment, Sims said. Initial efforts were rocky. One year when kids divided their days between co-ed and single-sex classes, "They would think it was party time [in the co-ed classes], because they got to socialize with the opposite gender," Sims said.

Now, students are split up for most classes in boys' and girls' academies, though they mingle in the hall and at recess. Sims says the academies have helped Battle Creek create a positive culture and that disciplinary problems have diminished.

For fourth-graders in two single-sex classrooms at Sunrise River Elementary in North Branch, "the academic gains have been very consistent with their peers" in co-ed classes, said Principal Jason Hartmann. But the single-sex classes provide a choice that remains popular, attendance rates are better and kids seem to gain confidence in them, he said.

'Outside the box'

But in some other districts such as Edina and Burnsville, similar classes have died out within a few years, with officials citing low interest, external factors such as budget woes, or lack of evidence that separating girls and boys boosted achievement.

At schools such as Lucy Laney, the need to raise achievement was the driving force behind the change. The K-8 school was identified as one of the state's lowest performers, and school officials were offered federal funding in exchange for major reforms.

Last year, the Minneapolis school began splitting up girls and boys in grades six to eight.

"We had to start thinking outside the box," said assistant principal Mauri Melander. "The things we were doing were not working."

Sarah Lemagie • 952-882-9016