Finding this hard now my dd is a teen - she wants to wear micro-minis and make-up etc - she knows she looks good and enjoys the positive attention from boys as well as other girls. As a feminist, I don't want to tell her she can't wear what she wants because people might get the wrong idea etc ie akin to blaming the victim - but as her mum, I want her to be safe, and not give boys the impression she's up for all things sexual because she isn't.
It was easy for my mum's generation - she could just say that 'nice girls don't' and that basically sex was not nice and best avoided until obligatory in marriage - not correct,but at least unambiguous. 'Nice girls' also didn't wear short skirts etc. But trying to give positive messages about sex whilst also making it clear it's best left till lots older (she's 13) and about the right to dress to please herself whilst also understanding that other people will judge you on the basis of what you wear, is a rather tricky balance to express.