Explain to her through stories and examples that the value of a human being does not depend on how she looks or the clothes one wears but on the qualities one has and the contribution one can make to the world.
That friends whose friendship depends upon the dress are not worth her friendship.
And that what matters most how much she can progress and learn in the school is what will help in the future and bring in the right kind of friends.
Affectionately,
Alok da.