London, England - Lawyers for a 12-year-old girl say her school is "irrational" in not letting her wear a full facial veil because her three sisters were allowed to do so.
They told a judge at the High Court that the girl's elder sisters went to the same school in Buckinghamshire and had worn the niqab.
The school, which is not being named for legal reasons, says the niqab does not comply with its uniform code.
The girl's family are seeking a judicial review of the school's ruling.
A judge heard that the girl's sisters had all attended the same school in the past - two of them under the present head teacher - and had all worn the niqab with no objections.
The niqab covers all of the face except for the eyes.
The three young women had participated fully in school activities and the school was "very supportive of them as devout Muslims and the way they expressed their faith," said Dan Squires, lawyer for their sister.
It was not rational to stop the youngest girl wearing the niqab, he told Mr Justice Silber.
Legal costs
The girl is protected by an anonymity order, as is her Buckinghamshire school, and was only referred to as 'X' in the case.
Her lawyers also argued that the school's ruling was breaching her right to freedom of "thought, conscience and religion" under Article 9 of the European Convention on Human Rights.
The school says the full face veil does not comply with its uniform code because it hampers communication.
The girl involved has not been excluded from the secondary school but has been asked not to come wearing the niqab.
Buckinghamshire Council had warned that the school might be forced to back down because of the high cost of the legal proceedings.
But The Muslim Educational Centre of Oxford has said it will raise funds for the school. Its chairman Taj Hargey said he wanted to step in to show that not all Muslims agreed with the niqab.
Uniform code
The pupil, who joined the school in September, has not been in lessons since October. She has had some tuition at home.
Last year, the Law Lords ruled that a Luton school was justified in banning the jilbab, a long gown, but it took a lengthy legal battle before this was established.
Government guidance says schools can decide their own uniform code in consultation with the community.