Principal Thierry Vieusses, who took down the tree in Lagny-sur-Marne last week when pupils challenged it, reversed his decision after the local mayor and French media ridiculed his reading of a new law on secularism in state schools.
The row echoed a similar controversy early this month when schools in the northern town of Coudekerque-Branche cancelled their annual distribution of chocolate Saint Nicolas figures because the foil showed him wearing a mitre with a cross.
"The tree was a symbol of year-end festivities long before Christianity existed," Pascal Pagny, mayor of the town just east of Paris, told Europe 1 radio on Saturday. "It is completely secular and pagan."
The tradition of decorating evergreen pine trees during the shortest days in late December dates back to ancient times and was taken on by Christians only much later.
Vieusses removed the tree from the entry hall after two girls challenged it as a violation of the secularism law passed this year mostly to stop Muslim girls from wearing headscarves.
The Christmas tree is completely
secular, the town mayor said
Other pupils criticised him for giving in to a small group they said was misinterpreting the law and pressed to have the tree restored. Vieusses finally reversed his decision after meeting pupils, parents and teachers.
In an open letter, Pagny accused Vieusses of embarrassing the town and "giving the impression that, in Lagny-sur-Marne, a handful of extremists can easily make the authorities give in".
The Christmas incidents were the latest examples of the surprise side-effects of the disputed secularism law.
A Paris woman, who often helped out as a volunteer monitor on school outings, was barred recently because she was wearing a headscarf, even though the law only applies to pupils.
Three Sikh boys were barred from their school in a Paris suburb for refusing to take off their turbans.
Forty-three girls have been expelled from French schools this year for refusing to take off their scarves, which they say Islam requires them to wear. The official French Council of the Muslim Faith advised them to respect the law.