Christian crosses are actually forbidden in French school (from what I read). I don’t know if anybody ever got removed from school from it, but the rule is there. I can’t talk on what is against or not French constitution as I am not qualified to do so (not even for my own country), but I trust that if that’s the case, courts will determine that.
A final remark, being Muslim is a choice, is not a birth condition nor a race (or ethnicity). This means that at most you can talk of religious discrimination, not racism. Coincidentally religious discrimination is very common in very religious countries (including Muslim countries), both towards other religions and even more against atheists or apostates.