Reb Moshe, Even Haezer (1:69) last paragraph:
“But there is another issur for women from the din of Das Yehudis not to behave with pritzus from Kesuvos 72. But from this aspect, it is only when she alone dresses that way. But when it is the derech of all the women in her city to dress that way, it is not shayach to consider it as pritzus. And it makes no difference whether the original custom of that city developed because of intentional pritzus, since at any rate because it is the derech of their dress and walking around, one cannot consider it as an act of pritzus and to forbid it to them, rather from a derech chasidus for extra tznius, and they should be blessed (Tavo aleihem bracha).”
So Oomis is 100% right. A lack of tznius does not equate to pritzus. Note that most places in shas, tznius is used as an extra credit type of context. For example, the story of Hilni Hamalka in beginning of Sukkah which says that her sukkah was compartmentalized for extra tznius. The gemara orginally asked why a queen should have a compartmentalized sukkah, not a nice big open one. The gemara answers it is possible she had it for tznius. The gemara is not implying she is required to have such a sukkah, or else what was the hava amina.
As far as the story with my daughter, she had a skirt that everybody thought was fine, except maybe the pockets or zipper were in the wrong side, so I told her it was improper, because the camp list said that is wrong. (I was naive back then). She wanted to wear it. I made a big deal over it. Now I regret it, because how in the world did such a thing become halacha, and why did I aggravate my daughter and cause bad feelings between us over this. Of course there is nothing wrong with that. It just may not fit into the arbitrary cookie-cutter definition of what Bais Yaakov girls are supposed to look like.