With all this talk of what tznius is not about, I thought I should take a shot at explaining what tznius actually is all about. And I figure I should start with the problem I have with the Charedi attitude toward tznius, in the way they educate girls. Tznius in most Bais Yakov schools is just a bunch of extreme rules that the teachers over-emphasize at the cost of actually teaching girls what tznius is really all about. It's about skirt lengths and thick stockings, and about getting yelled at for not complying with what they claim are the halachic requirements of tznius, when they are really just chumras.
So what is tznius really about? I think what tznius is really all about is focusing on the internal aspects of a person, not on the external aspects. The purpose of women dressing modestly is so that when you see the woman, you are not distracted by her appearance, but instead can focus on her internal qualities. This I believe is the meaning of the statement "kol kvuda bas melech pnima," that the honor of a woman is in her ability to draw attention to her internal qualities, not her outer appearance.
So what is the result of Bais Yakov schools teaching that tznius is all about these stupid rules and nothing else? Just have a look around Flatbush and the 5 Towns, veha-mayvin yavin (no pun intended). And if you are not so fortunate, I will fill you in. Basically, many women follow the rules, but still manage to dress extremely provocatively. This is of course the result of being taught that tznius is just a bunch of rules.
So how do you teach girls what tznius is really all about? Maybe teachers can be role models themselves of how to act tznius. Maybe they could focus on the internal qualities of their students, instead of their skirt lengths.