Sometimes it’s hard to believe we’re living in the 21st century. Thanks to a tide of distorted “brain-based” education, some 750 public schools around the United States have been segregating boys and girls into single-sex classrooms that sound like the old woodshop and home economics classes of the 1950s.

Consider Middleton, Idaho, where elementary teachers electronically amplify their voices in all-boys’ classrooms but not in girls’ classrooms, based on absurd extrapolations about male-female hearing differences. Middleton teachers also reportedly encourage boys to run and play before exams, whereas girls are led in “calming yoga exercises” based on fabricated differences in their brains’ stress response systems.

Gender segregation has been allowed to flourish for nearly a decade in U.S. public schools. The good news is that the Department of Education’s Office for Civil Rights has finally taken notice, and this week it issued guidelines to prevent schools from using biological differences as a basis for teaching boys and girls differently. The department’s guidance may be the first step toward battling back the gender distortions that have flooded K-12 education for years now.

For instance, the “busy boys and little ladies” title comes from teacher training materials in Florida, a hotbed for single-sex classrooms and whose teachers are mandated by law to receive professional development in gender and education. This doesn’t sound like a bad idea, until you learn that school districts are basing their training on the gender musings of Michael Gurian, a pop psychologist who lacks any training in neuroscience or education. In the Hillsborough district that includes Tampa, nearly $100,000 of taxpayer money has gone to the “Gurian Institute” and other trainers, who advocate blatantly stereotypical practices salted with just enough distorted claims about the brain and hormones to fool teachers into thinking they are scientifically based.

Here’s a typical brain-sex factoid touted by Gurian: “Boys come out of the womb with a formatting for non-verbal, spatial, kinesthetic activity on the right side of the brain. In the areas where girls’ brains come out ready to use words, boys’ brains come out ready to move around, kick and jump.”

It all sounds so sensible—right on target with most gender stereotypes and therefore perfect justification for educating boys and girls differently. Except that none of it is true! There is no differential “formatting” of boys’ and girls’ brains, and no difference in the brain areas men and women dedicate to verbal or spatial abilities. Nor does testosterone—the favorite go-to hormone for Gurian and other gender segregationists—ramp up boys’ math skills or suppress their language development, as teachers in Florida have also heard in their training.

It’s bad enough to see teachers amplifying gender stereotypes, but it’s truly distressing to hear students themselves parrot false beliefs about boys’ and girls’ brains and abilities. In Tampa, a pair of single-sex middle schools, Ferrell and Franklin Academies, actually posts home page videos of girls boasting about their superior frontal lobes and ability to read facial expressions and boys expounding on their brains’ better visual and spatial processing. The implication of girls or boys articulating, respectively, “We’re good at emotion” or “We’re good at spatial processing” is the unspoken, but powerful corollary: “and we’re bad at thinking” or “we’re bad at talking.”

If separate and unequal classrooms sound illegal, it’s because they probably are. The Department of Education’s action was triggered by several complaints, including some filed by the American Civil Liberties Union, which has challenged single-sex programs in Florida, Texas, and other states. The move is part of the group’s larger “Teach Kids, Not Stereotypes” campaign that urges stronger enforcement of Title IX and the Equal Protection Clause of the Constitution.

Title IX regulations expressly forbid single-sex education that is based on “overly broad generalizations about the different talents, capacities, or preferences of either sex.” With its new guidance, the Department of Education appears to finally agree that these pseudoscientific claims about boys’ and girls’ hearing, vision, stress response, and cognitive abilities qualify as “overly broad.” The problem with such generalizations is that there are plenty of boys and girls who don’t conform to them—girls who are very physical or good at math, and boys who are very sensitive or good at reading—and are therefore being marginalized in such environments.

Of course, not all single-sex schools rely on brain sex differences for justification. But as Rebecca Bigler and I wrote earlier in Slate, the very fact of segregation of any type accentuates group differences. In children, especially, research finds that gender segregation exaggerates their beliefs in hardwired, immutable differences between the sexes. So while girls or boys may love their single-sex classes and even feel temporarily empowered in them, the loss of opportunity to work with members of the other sex ultimately fosters a distorted belief in gender difference that restricts all children’s potential.

Considerable research has now proven that single-sex education does not produce better academic outcomes than co-education. Which raises the question: Why have gender segregation at all in K-12 schools? We live in a diverse, pluralistic society, where schools need to better prepare boys and girls to work together, raise families together, and share leadership in the future. Most Americans now abhor racial segregation in schools, and by similar logic, gender segregation seems a poor way forward for today’s young people, especially when justified by pseudoscience.