Clearheaded (Philosopher) brings up a good point. There are TWO reasons for Tznius.
One reason (which should be tought more) is the “Hatzneya Leches” aspect, and how everyone (men and women) should act and dress in a manner befitting a Torah Jew.
The second reason (which I am afraid is more stressed) is the “you can’t do that/wear that”, which bases off of the technical aspects of Tznius (like the issur for a man to see a woman who is not Tzanua (elbows knees and neckline)). Girls are tought what is (or is claimed to be) Assur vs. “How to dress as befitting a Bas Yisroel”.
Teaching what is Assur is what drives girls crazy (and not wanting to be Tzanua). The first aspect would teach them self-esteem and be a better lesson for life. Judaism is a positive religion, and going negative could drive girls off.