WASHINGTON, March 3 - The Bush administration has proposed regulations giving public school districts new freedom to create same-sex classes and schools, as long as "substantially equal" opportunities are also provided for the excluded sex.

Supporters and critics alike said the proposed changes represented a major reinterpretation of antidiscrimination laws, some 50 years after the Supreme Court discredited racial segregation in "separate but equal" schools as inherently unequal, and 30 years after Title IX extended the concept to sex.

Under the new rules, educators could create new schools or classes exclusively for students of one sex, and no longer have to demonstrate that they were doing so to remedy past discrimination. In addition, rather than having to ensure that students of the other sex were receiving the same opportunities in a single-sex setting, the new rules would allow them to provide those opportunities in a co-ed setting.

Federal officials said the changes would allow schools to offer a greater variety of educational options to parents, and said that under no circumstances would children be required to attend same-sex schools or classes. The regulations would take effect after a 45-day comment period.

"We are not advocating single-sex schools and we are not advocating single-sex classrooms," said Kenneth Marcus, who oversees the Office for Civil Rights in the federal Education Department. "We understand that co-ed remains the norm. We are simply trying to ensure that educators have flexibility to provide more options."

The new regulations drew immediate fire from some women's and civil liberties groups, who said they were in violation of Title IX, the landmark law that prohibits sex-based discrimination in schools. Even some supporters of the new rule suggested it stood on shaky ground.

"This is not an evolutionary position that they've taken. It's revolutionary," said Randy Tucker, vice president of the board of the National Coalition of Girls Schools. "It's a longstanding position that separate but equal was not a valid standard for education, whether it was by race or gender."

Mr. Tucker said the new rules were certain to face legal challenges.

Susan Aspey, a spokeswoman for the Education Department, said the proposed changes "include ample safeguards to ensure nondiscrimination," and could withstand legal challenges.

For the moment, the changes affect but a single small thread in the tapestry of public education. There are currently 24 same-sex public schools across the country, many opened in the last two years, and 93,000 co-ed public schools. The issue gained new momentum with the passage of the No Child Left Behind law, when women senators from both parties came out in support of same-sex education, and asked the Education Department to draft guidelines to permit their growth by last May.

Research on the gains of children in same-sex vs. co-ed classrooms has been inconclusive. A 1998 survey of research by the American Association of University Women found no overall benefit to same-sex classrooms or schools, but some research since then has suggested that girls learn differently than boys, and that some students learn better when separated from the opposite sex.

The Bush administration has earmarked $297 million each year for "innovative programs" in education, including single-sex schools. Ms. Aspey said the department did not track how much of the money was going to same-sex instruction.

In recent years, the attitude toward same-sex education has undergone major change, said Tom Carroll, the founder and chairman of Brighter Choice Charter School for Boys and Brighter Choice Charter School for Girls in Albany. "Before the department was trying to stamp out single-sex instruction, and now they've thrown the door wide open for schools and districts to offer it."

That is precisely what has alarmed women's rights advocates like Marcia Greenberger, co-president of the National Women's Law Center, which spent much of last year battling the Education Department's proposals to rewrite Title IX regulations that protect women's sports programs. In the end, the regulations remained intact.

Ms. Greenberger said her organization would consider challenging the new guidelines in court. She called the changes "a very serious attack on the most fundamental principles underlying protections against sex discrimination under Title IX and under our Constitution," and said they would "set back progress that women and girls have made for over 30 years."