discriminating school dress code

This week at my high school is spirit week and today's theme is "formal" sounds fun! So my good friend is happy to participate and has her black dress from homecoming. Upon arriving to school she was told by administration to "COVER UP OR CHANGE" below I have a picture of the sheer back of her dress that was "so distracting and revealing" and a couple others that are apparently ACCEPTABLE articles of clothing to wear!!