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POMPANO BEACH, Fla. — In one third-grade classroom, the walls are bordered by cheetah and zebra prints, bright pink caddies hold pencils and glue sticks, and a poster at the front lists rules, including “Act pretty at all times!”

Next door, cutouts of racecars and pictures of football players line the walls, and a banner behind the teacher’s desk reads “Coaches Corner.”

The students in the first class: girls. Next door: boys.

Single-sex education, common in the United States until the 19th century, when it fell into deep disfavor except in private or parochial schools, is on the rise again in public schools as educators seek ways to improve academic performance, especially among the poor. Here at Charles Drew Elementary School outside Fort Lauderdale, about a quarter of the classes are segregated by sex on the theory that differences between boys and girls can affect how they learn and behave.

Teachers “recognize the importance of understanding that Angeline learns differently from Angelo,” said Angeline H. Flowers, principal of Charles Drew, one of several public schools in Broward County that offer some single-sex classes.

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The theory is generally held in low regard by social scientists. But Ms. Flowers notes that after the school, where nearly all students are eligible for free or reduced-price lunches, started offering the classes two years ago, its state rating went from a D to a C. Similar improvements have been repeated in a number of other places, causing single-sex classes to spread to other public school districts, including in Chicago, New York and Philadelphia.

The federal Education Department says there are about 750 public schools around the country with at least one single-sex class and 850 entirely single-sex public schools. Although government figures are not available for earlier years, the National Association for Single Sex Public Education estimatedthat in the 2004-05 school year, 122 public schools offered at least one single-sex class and 34 public schools served just one sex.

Critics say that there is little evidence of substantial differences in brain development between boys and girls and that dividing children by gender can reinforce entrenched stereotypes.

Rebecca Bigler, a psychologist at the University of Texas, said that segregating by sex — or any social category — increases prejudice based on stereotypes.

“You say there’s a problem with sexism,” Ms. Bigler said, “and instead of addressing the sexism, you just remove one sex.”

That worries the American Civil Liberties Union, which this year filedcomplaints with the Education Department against four Florida school districts, accusing them of violations of federal civil rights law and of using “overly broad stereotypes” to justify separating girls and boys into different classrooms. The A.C.L.U. also filed a complaint in Austin, Tex., against two new single-sex middle schools, and has pending complaints in Idaho and Wisconsin and a nearly two-decade-old complaint in New York. Lawsuits in Louisiana and West Virginia have resulted in single-sex classes there reverting to coeducation.

Advocates of single-sex classes often cite the struggles of boys, who persistently lag behind girls in national tests of reading comprehension and are much more likely to face disciplinary problems and drop out of school. Educators also argue that girls underperform in science when compared with boys and benefit from being with other girls. And school officials say that children can be easily distracted by the opposite sex in the classroom.

This week, in response to the A.C.L.U. complaints and the growth in single-gender classrooms, the Obama administration is issuing guidance for school districts.

Schools may set up such classes if they can provide evidence that the structure will improve academics or discipline in a way that coeducational measures cannot. Students must have a coeducational alternative, and families must volunteer to place their children in all-boys or all-girls classes.

But the guidance says that “evidence of general biological differences is not sufficient to allow teachers to select different teaching methods or strategies for boys and girls.”

“I am very concerned that schools could base educational offerings on stereotypes,” Catherine E. Lhamon, assistant secretary for civil rights, said in an email. “No school should be teaching students to live down to diminished expectations for who they can be.”

Asking schools to demonstrate academic improvement could prove difficult.

Over all, research finds that single-sex education does not show significant academic benefits — or drawbacks. Janet Hyde, a psychologist at the University of Wisconsin in Madison who analyzed 184 studies covering 1.6 million children around the globe, said the studies showing increased academic performance often involved other factors that could not be disentangled from the effects of the single-gender component.

Supporters say girls have more in common with other girls — and boys with other boys — than with the opposite sex of the same age. “Yet we segregate on the basis of age, not based on any evidence,” said Leonard Sax, a pediatrician and author of several books on gender differences, including “Why Gender Matters.”

According to the A.C.L.U.’s complaint in Broward County, the district relied on materials from Mr. Sax as well as from the Gurian Institute, a Colorado-based business founded by Michael Gurian, the author of several books, including “Boys and Girls Learn Differently.”

The training materials, the complaint says, noted that “gently competitive lessons may be more impactful for boys” and that “lessons that incorporate emotions and emotional vocabulary” may have more impact for girls. Teachers were also advised to be “more tolerant of boys’ need to fidget or girls’ need to talk during class.”

Many of the schools that offer single-sex classes have struggled with student academic performance and are in high-poverty neighborhoods dominated by racial minorities.

“Parents really are starving for better options,” said Galen Sherwin, a senior staff attorney with the A.C.L.U. Women’s Rights Project. “And oftentimes school districts sell these options as the solution with inflated, unsupported supposed evidence.”

On a recent morning at Dillard Elementary in Fort Lauderdale, where 98 percent of the students are black and nearly all come from low-income families, MeLisa Dingle-Mason, a third-grade math teacher, echoed some of her training.

“I am able to push them to their level and include sports and different things,” she said of the boys she teaches for part of the day before swapping with a reading and social studies teacher to work with girls. She added that she liked to turn math sessions into games because boys “like competition.”

The boys in her class appeared busy and eager to work. Jaheim Jones, 8, said he preferred a girl-free zone at school because girls are “bossy.”

Down the hall in a third-grade reading and social studies class, Ruth Louissaint, who was overseeing all girls at the time, showed a crate she kept in a storage room of fuzzy pastel blue sweaters for girls, saying they were more likely to feel cold than boys. For spelling and vocabulary lessons incorporating physical activity, Ms. Louissaint brought out hula hoops and small rubber balls for the girls. The boys would get yo-yos, bats and badminton rackets.

She said she taught the same curriculum to both but changed background details. So when playing music in class, for example, she tends to put on Michael Jackson for the boys, switching to more soothing music by groups like Enigma for the girls.

Angela Brown, the principal at Dillard, said boys in single-sex classes had better attendance than those in coeducational classes as well as better scores on state reading and math tests. But the biggest improvement was a decline in disciplinary infractions and bullying.

“Boys are trying to impress girls, and girls are trying to impress boys,” Ms. Brown said. “And we have removed that variable out of the way.”

Throughout Broward County, an external evaluation by Metis Associates, a research firm, found that after two years of offering single-gender classes in five schools, nearly half of the students experienced a decline in disciplinary referrals, detentions and suspensions compared with a year earlier.

A preliminary analysis of state test scores showed that about three-quarters of the students enrolled in single-sex classes improved their percentile rankings on reading scores, while close to 70 percent of elementary students in single-sex classes raised their scores in math.

Broward County officials said that although the district added two new single-sex options at a middle school this year, administrators were not planning to expand rapidly.

“We are not just doing this randomly, ” said Leona Miracola, director of innovative programs for Broward County Public Schools, adding that the district takes compliance with federal law “very seriously.”

Shenilla Johnson, 9, a third grader at Charles Drew, considers an all-girls class a boon. Boys, she said, “annoy you.”

Without them, she said, “we get to learn new things.”