Just to make my previous point clear: Of course a private school is free to set its own standards, including dress. If the director tries to impose standards to the family of the student, the family is free to accept or to refuse. (I believe this is plainly wrong, even on a religious level, as others have developed, in terms of relations between parents and children, but this is NOT the issue.) What IS at stake HERE is different. The families concerned here are WILLING to follow the schools' standards very strictly, and nevertheless, this WON'T help them to gain admission for their children to the school (thus #42, #50 and all those who think that it is just a question of 'internal rules' that are not well enough understood by non-haredim are just wrong). So the last paragraph of Kashti's article is well documented.
from the article: Ultra-Orthodox school bars girl over her 'mother's immodesty'