The principal at the Léo Lagrange school in Charleville-Mézières decided that the black maxiskirt the girl wore ran afoul of the rules and wrote to the girl’s parents that their daughter had been sent home to change into more appropriate attire.

Ms. Ray called it “really an excessive interpretation” of the law.

The collective has documented 130 similar cases across France since January 2014, and Ms. Ray said they were becoming more frequent. The cases often involve skirt length, she said, but schools have also objected to sweaters or to headbands that they say are too broad and are meant to evoke head scarves.

Because the girl in Charleville-Mézières is a minor, she has been identified in French news reports by only her given name and initial, Sarah K. Her case points to the difficulty in enforcing the French policy of laïcité — roughly, secularism — which strives to keep religion strictly out of government and the public sector. It has been invoked in recent years to restrict the places where Muslim women can wear clothing concealing their bodies or faces.

The case also illustrates the gap between the ways French officials and Muslims have understood the secularism rules.

Sarah’s mother told the French magazine L’Obs in an interview posted online Wednesday that although the family is observant, the parents never asked Sarah to wear a head scarf, and that several of her five older sisters did not wear one. “About a year ago, she started to wear a veil, as I do,” the magazine quoted the mother as saying. “But every morning when she gets to school, she takes it off because she knows it’s forbidden.”