Apparently what happened, according to the principal, was that all the Muslim girls in the area decided that they would all wear the same style of long black skirt.
The unspoken part of the story is that over the last couple decades, if the French Muslim girls in a rough neighborhood all decide to do one thing to show they’re Muslim, a lot of Christian and Jewish and none of the above girls get beaten up and harassed by the French Muslim “youths,” until and unless they do the same thing. Doing the same thing is seen as a sign of proper submission to Islam and a sign of Islam’s eventual conquest, and therefore, much glee ensues.
In this school, apparently a little farther away from the rough neighborhoods, there was only one Muslim girl. But in context, wearing the skirt of the movement was pretty similar to wearing a gang sign, or telling all the other girls in school that they are going to get beaten up and harassed. So French secularism does have its problems, but in this case there is meaning behind the madness.
However, there does seem to be some question as to whether the principal was wrong about the Muslim girls setting this up as a provocation, in this specific case. (This would be analogous to misinformation about just what constitutes gang signs, which is pretty prevalent in small towns that don’t have much gang activity.)