GAW – that might work as it teaches why you HAVE to do it, which might work when it comes to men, but I think that the yetzer hara has plenty of tricks to get us away from doing what we HAVE to do.
The thing to do is to teach a girl sensitivity about how tznius is important to her as a person and then she will hopefully come to WANT to be tznius. There is little ammunition of the yetzer hara for our WANTS.