Tzippi, I personally think its the fact that tznius is so emphasized in schools that it becomes background noise. People start to ignore it and do what they want. A BT friend of mine told me that if she had learnt tznius the way her daughters were being taught, she would have run away from the religion. She was taught similiar to what clearheaded posted.
But I do want to reiterate, most frum women I know dress for women and not for men. Teenager years may be different (I still think more dress for women than men).