The fey may be averse to cold iron, but an aversion to rust is probably just a personal thing.
More interestingly though, why would this random student, presumably native British of some sort, address Chise by her family name? Not that it is unheard of here, but it is usually formal to the point of condescension to do it here, at least without a title. The teachers might do it, but it isn't generally something students would do to each other unless they had some sort of animosity.