Is the Concept of Respectable and Respectful Dress Completey Subjective?

I had written a kind of long post about the whining that is going on about young women's clothing, and the arguing about whether telling them they they could save themselves a lot of heartache by dressing  less provacatively is "slut-shamming" and "victim-bashing," but I've put it on hold...

A friend of a friend is all het up because her 12-ish daughter was made to cover up because her school outfit was brandishing bra-straps.

Is this a feminist issue?
Is that really a discriminatory rule? That your underwear can't show?

Is that a random "fashion" rule of the sort that kept male and female tennis players alike in floor length wool?

Or am I turning into my grandmother -- you can't wear blue jeans into the "city"!