To be fair, whether you disagree with it or not, this is the spirit of the law: ostensible religious symbols are not allowed in public schools (and more generally in republican institutions). So yes, one symbol is more visible than others, and that makes it somewhat unfair, but the "visible" portion is realistic. You can't ask teachers to check under student's clothes for them.

I will agree though that France has also been very reactionary towards its Muslim population and laicity has been a convenient tool to bash them. But that doesn't invalidate the purpose of the original principle.