A Neighbor's Conflict
Answer: You are to be complimented for your discretion in regard to who are your daughter's social influences. You may be guilty, however, of being a bit hasty in judging your neighbor's daughter. Religious standards, whether in manner of dress or observance of mitzvot, are invariably shaped by the community in which one lives and the school one attends. Your question indicates that you reside in a community with high religious standards and that your daughter attends a school with such standards. Experience has shown that families joining a community such as yours seek to conform to a higher standard for themselves and their children and succeed in doing so.
Tell the principal the positive aspects of the family as you see them and present accepting the girl as a challenge to the school and your community to enable this family to grow.