GREENSBORO, Ga. — Nearly four decades after this rural Georgia county stopped segregating its schools by race, it wants to divide students again — this time by sex. Greene County is set to become the first school district in the nation to go entirely single-sex, with boys and girls in separate classrooms — a move born of desperation over years of poor test scores, soaring dropout rates and high numbers of teenage pregnancies. “At the rate we’re moving, we’re never going to catch up,” Superintendent Shawn McCollough told parents in an impassioned speech last week. “If we’re going to take some steps, let’s take some big steps.”

This pine-shrouded county of about 14,400 people between Atlanta and Augusta has in recent years become a magnet for retirees moving into luxury developments along the shore of Lake Oconee.

But the vast majority of longtime residents — and most of the 2,000 students in the county’s schools — are black and working class.

Mr. McCollough pointed to research showing that boys and girls learn differently and said separating them will allow teachers to tailor their lessons. Also, boys won’t misbehave as much because they will no longer be trying to impress the girls, and the girls will be more likely to speak up in class because they won’t be afraid to look smart in front of the boys, he said.

The school board’s move to radically overhaul the system next fall has angered parents, students, and teachers, who say they weren’t consulted. And one of the nation’s foremost proponents of single-sex education warned that the board has gone too far.

The measure, approved two weeks ago, applies to the high school, the middle school, and both elementary schools. It exempts only the preschool and a charter school, which is public but operates independently.

“I am outraged,” Tammi Freeman, who has two children at the high school, said. “I am disgusted. It’s making our county look like our kids are trouble when they’re not.”

Leonard Sax, head of the National Association for Single Sex Public Education, said that while single-sex schools and classrooms are on the increase, he knows of no other community that has converted its entire school system. He called the move illegal.