Women teachers seeking a headship will continue to hit education's glass ceiling because ministers have decided schools will not be covered by key parts of the most significant piece of sex equality legislation for 30 years.
A government consultation paper to be published next week is expected to exempt schools from new rules which will place specific duties on public-sector bodies to tackle discrimination.
The fear is that single-sex schools and lessons could be at risk if anti-discrimination legislation is applied.
Equality experts say the decision is a missed opportunity to improve women's promotion prospects, reverse the decline in the number of male primary teachers and tackle gender stereotyping in the opportunities given to boys and girls.
Male primary teachers are about four times more likely than women to become a head while male secondary teachers are three times more likely to get the top job.
A study published by the National College for School Leadership in March found that half of women secondary heads had to fight sexism to win promotion.
Reasons given to women who failed to win headships included wearing too much nail varnish or gold jewellery and being too short.
Since 1991, the proportion of male primary teachers fell from 22 per cent to less than 14 per cent.
John Bangs, National Union of Teachers head of education and equal opportunities, accused the Government of leaving a gaping hole in schools'
equality duties. He said ministers had an unfounded fear that the new duties could undermine single-sex teaching. "I would be very concerned if education was left out," he said.
Jenny Watson, acting chair of the Equal Opportunities Commission, said schools should not be exempted.
She said: "This is the most significant piece of sex equality legislation for 30 years.
"The gender duty gives us a massive opportunity to tackle occupational segregation and make sure boys and girls have the same range of choices to develop their potential free from other people's outdated assumptions."
Girls are often pushed towards vocational options and ultimately careers in often low-paid areas such as childcare and hair-dressing while boys are steered towards construction and motor mechanics.
The consultation paper will set out the specific duties required of public bodies under the Equality Bill which will be reintroduced to Parliament this autumn.
Specific duties to address the gender pay gap and ensure services take account of the different needs will not apply to schools although they are expected to be covered by the general duty to promote equality.