The school has a dress code and they should be entitled to enforce it. If the girl does not like the dress code let her go to a private school or go back to her own country. Why should a school not be allowed to have a dress code? If no jewelry is part of the dress code then crosses should not be allowed. This has nothing to do with religion.
The United States is a country founded on the principles of Freedom, not on authoritarian whims. If the dress is not a minimal mirror of the general public then it is not appropriate. In this case, the headgear is cultural and is intimately connected to their historical religion. It is part of the tradition of their religion.