The move broke the International Covenant on Civil and Political Rights, the UN Human Rights Committee said.
Its decision follows a complaint filed in 2016 by a French national born in 1977, whose lawyer does not wish her name to be published.
The woman was on a professional training course for adults in 2010, and had passed an interview and entrance test.
But the headmaster of Langevin Wallon high school in the southeastern suburbs of Paris refused to let her enter because of a ban on wearing religious symbols in public educational establishments.
The UN committee said “prohibiting her from participating in her continuing education course while wearing a headscarf constitutes a restriction on her freedom of religion in violation of the treaty”.
The committee’s decision was adopted in March but sent to the woman’s lawyer on Wednesday.
“This is an important decision which shows that France has work to do in terms of human rights and in particular on the issue of respect for religious minorities, and more particularly the Muslim community,” her lawyer Sefen Guez Guez told AFP.
The issue of religious symbols and clothing is an ongoing one in France, where laÏcité (secularism) rules mean that all overt religious symbols – including the Muslim headscarf – are banned for staff and pupils in public buildings including government offices and schools.
The most recent flare-up over laïcité rules involved local authorities in Grenoble, who had voted to allow the full-body swimsuit known as the ‘burkini’ in municipal swimming pools. Their decision was contested by the interior minister, Gérald Darmanin, who referred the decision to the administrative court, which blocked the Grenoble authority’s decision.
The burka and niquab are banned outright in France, while the Muslim headscarf can be worn in all public places apart from government-run buildings. Public officials who represent the state – such as police officers – are also banned from wearing it while at work.