I personally have never seen a uniform policy that restricts headdresses. (and I have read many) I have never seen a nurse wearing a headdress but I have seen physicans wear them with no comment on anyone's part. I think that not allowing a student to wear her hijab could constitute a serious discrimination problem as it does not interfere with her work or performance in any way. I realize it is not as obvious, but I see many nurse wearing their cross and star of david, not the same nurses, of course.