Interestingly, if this was a dispute in the US, the courts would probably side with the school. There is a fairly well developed body of law in this country that says religious schools are entitled to enforce basic tenents of their religion in their hiring and firing practices, and generally US courts will not interfere with them when they do that. The last such case I recall involved an employee fired by Brigham Young University because, even though he was a Mormon, he did not tithe to the Mormon Church.
Interesting point. I would guess, though, that this Islamic school is heavily subsidized by the Dutch government. I would also guess that the French headscarf ban will be more of a precedent for Dutch courts than US law.