I think some of them at least do it as a sign of protest against the French society. It's normal when you are a teenager to want to defy your teachers and the school system. Of course this is somewhat used by religious preachers to gain power. But these types of law will not make the girls want to wear these clothes less, and may pressure them more into religion. Also, the extreme religious kids don't usually go to the secular public school, there are religious schools in France. The true response to this issue is more social workers, more information about women's rights for young girls, actual solutions for women who are attacked or pressured into religion, and a better economic and social integration of these population, regardless of the clothes they are wearing.