President Emmanuel Macron says French students will be barred if they show up for school wearing long robes.
He says authorities would be stubborn in enforcing the new rule when classes resume next week.
Education Minister Gabriel Attal announced at a news conference four days ago that robes worn mainly by Muslims, known as abayas for girls and women and khamis for boys and men, would be banned in the coming school year on Monday.
The education minister described girls and boys wearing the robes in secondary schools as "an infringement on secularism", a foundational principle for France.
The framework for the ban is a 2004 law aimed at preserving secularism in French public schools.
The law prohibited Muslim headscarves but also applied to large Christian crosses, Jewish kippas and the large turbans worn by Sikhs. It passed after months of furore and marathon parliamentary debates.
The Education Minister said 14,000 educational personnel would be trained by the end of this year to deal with enforcement and other issues in upholding secularism.
The new rule has received worldwide criticism.