I am a pretty bold teacher and tell my girls they need to dress better, in fact I tell them to leave something to the imagination and now they will comment to me if I haven't said anything about another girl in class, they always say, well aren't you going to say something to so and so, her dress is short today.
Some days they come in and say, look I remembered to put a cami under my top today...its pretty funny.
I am glad your sister had fun and she was able to get a dress that worked.