A Cumbrian family is one of three taking High Court action against the Government, for leaving out "non-religious world views" from the new Religious Studies GCSE.
They are backed by the British Humanist Association, and they argue Education Secretary Nicky Morgan has taken an unlawful approach andis failing to reflect the pluralistic nature of the UK.
Changes to RS GCSE subject content were announced last February.
They led to complaints over the way priority has been given to religious views - in particular Buddhism, Christianity, Catholic Christianity, Hinduism, Islam, Judaism and Sikhism.
David Wolfe QC, for the families, told Mr Justice Warby the curriculum was being skewed, and there was widespread concern "about the secretary of state's failure to comply with her duty of neutrality and impartiality as between religious and other beliefs".
Earlier this year the former Archbishop of Canterbury Rowan Williams was among 28 religious leaders who urged the Government to rethink the current plans to leave out humanism in the new qualification.
Lawyers for the Education Secretary are arguing that neither statutory provisions nor the European Convention on Human Rights require equal consideration to be given to religious and non-religious views in the curriculum.
They submit that, although some schools rely on the RS GCSE to discharge their duty to provide religious education at key stage 4 for 14-16 year olds, provision has been made for non-religious beliefs to be studied and what is in a school's curriculum is a matter "for local determination" by individual school authorities.
The families seeking judicial review include one from Cumbria and one from Kent and cannot be identified.
The third family is Kate Bielby, of Frome, Somerset, and her daughter, Daisy.
The law is clear that when teaching about religions and beliefs, schools should follow a broad and balanced syllabus which includes both religious and non-religious world views like humanism on an equal footing.