I do not understand why there is a rule against dressing MORE modestly than everyone else. What's the problem?
Should the school betelling the girl how to dress, and overriding family members telling her how to dress? I don't know about the brother, but the parents should be able to tell a minor how to dress.
The school in question, like many British schools, has a school uniform.
The veil for Muslim women is a relatively recent institution and is as much a political statement as an expression of religious modesty.