A Muslim free school has reportedly insisted staff, including non-Muslims, sign new contracts insisting they wear hijabs.
The Derby Telegraph reported that teachers were also concerned about gender segregation at the school, with girls reportedly asked to sit at the back of classrooms.
Al-Madinah school in Derby, which opened in 2012, is obliged as a free school to recruit 50% of pupils without taking religion into consideration. The school is currently under investigation by the Education Funding Agency for irregularities.
Sue Arguile, branch secretary of Derby National Union of Teachers, told HuffPost UK that the NUT was not challenging the school directly about the alleged sex segration, but had other "deep concerns' about what the school's founding principles were, how it obtained funding, non-qualified teaching staff and the education it is now providing, when compared to its original intention.
The Huffington Post UK has contacted the school for a response. The school has addressed concerns about the qualifications of its teachers and the departure of members of staff in a post on its website, entitled "Rumours and the news".
"Some staff have moved on, as they would do in any school, for a variety of reasons, and any administrative or contractual issues will be worked through.
"Al-Madinah is a pioneering school, the first of its type in the whole country, and not everyone wants it to succeed. Unfortunately politics have been allowed to interfere with education. Our only concern in the success of your children."
The school's founding headteacher Andrew Cutts-Mckay left after less than a year in post. He was quoted as saying, before the school opened, that it would "honour all faiths" and would have 50% non-Muslim pupils.
"I was drawn to the Al-Madinah School because of the ethos and culture that's going to be developed and introduced," he told the BBC in April 2012, as the school prepared to open.
"It's about developing a really effective school with high standards."
The paper reported that before the school was opened, it was under pressure from various local mosques to have a more strict Islamic ethos than what had been originally intended.
Arguile told HuffPost UK that five to 10 teachers at the school had come to the union with concerns about their new contracts. "The question is whether the school is continuing to provide the education which it received funding to provide, based on Muslim principles and not as a Muslim school.
"There's nothing intrinsically wrong with a school having a dress code if it's in place when teachers first sign the contract. You sign the contract, agree, or don't teach at the school. The problem is if one is introduced and teachers cannot object.
The school is a primary and secondary school, teaching just Reception and Year One, and Years Seven and Eight. It will eventually cater for 1,100 pupils, according to its website.
Arguile said the NUT had very little power apart from to advise its members on contractual matters. Free schools do not recognise the unions.
Andrew Copson, chief executive of the British Humanist Association, which campaigns against the setting up of religious free schools, told HuffPost UK he questioned the legality of contractual religious dress codes that apply to all employees regardless of their beliefs and of gender-based segregation in the classroom.
"To see them in a state school - an institution that should embody the highest principles of openness and equality is shameful.
"The fact that these practices occur are a symptom of the continuing failure of government to regulate effectively the practice of state schools with a religious character."