© GettyThe girl claims the wearing of masks could cause "long-term" health problems for children
She is suing the Tapton Academy School Trust, which runs schools in the Sheffield area, claiming mask-wearing "risks causing children serious harm".
The 12-year-old, who is exempt from wearing a mask at school, says pupils' physical and mental health are at risk.
The trust said it was following government guidance to avoid spreading Covid-19 among students and staff.
At a remote hearing on Friday, the girl's lawyers asked the High Court to grant an interim injunction preventing her school and the trust from making children wear masks.
Enforced 'aggressively'
Francis Hoar, representing the girl, told the court: "The school's policy risks causing children serious harm to their physical health and their mental health.
"If the trust had done its job properly... it would have gathered evidence and reached a view as to the effectiveness of this particular measure, but it has done no such thing."
Mr Hoar accepted that the girl did not have to wear a mask at school, but added that she was faced with a school environment in which the wearing of masks was enforced "aggressively".
Ben Bentley, representing the trust, told the court that "the trust policy is to encourage, not to enforce" the wearing of masks.
He said the trust had "quite properly" carried out a risk assessment and encouraged pupils and staff to wear masks, in part to help prevent transmission of coronavirus in the wider community.
He said 120 members of staff across its various schools, representing more than 10% of its workforce, had contracted coronavirus since the end of August 2020.
He added that the application for an injunction was brought by "one claimant in the school at which, we are told, there are 1,000 pupils".
Judge Roger ter Haar QC told the hearing he would give his judgement at a later date.
Comment: Masks are ineffective and, by restricting oxygen intake and fostering an unhygienic environment, they're harmful to health, moreover, they're unnecessary because children are at next to zero risk from the coronavirus. And so, with that in mind, why are schools forcing children to wear masks?