Girls usually know, when they dress, that a low-cut or a short seam is going to draw special attention. Even if they are "good girls" and have no intention of losing their virginity, girls know when their clothing is seductive. They may not think in those terms, but they know when their cleavage is going to be distracting.
But what will be distracting also depends on the time, occasion, and culture. What is acceptable in some warmer-climate cultures is not in a New England town.
The importance of modesty lies in whether a girl is objectifying herself. It's not really about whether men are going to be turned on – they need help learning to respect women as persons. Modesty is really about women being seen as objects, and that starts with their own self-image.
