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…

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.”