yeah you see you can't do the intelligent thing of just saying "you have to come to school with normal clothes on" which is literally what countries with a brain did. Also this is generally left to the school to set up. In France the law just says that a school is allowed to implement rules regarding clothing. Just simply think of P.E clothing. The law has a couple safeguard like you can't ban a specific outfit for being from another culture (obvious) and the rest is left for the school to decide