Long skirts and dresses, covering ankles and feet was the standard of the women's fashion before on the dawn of the 20th century when the Beyt Yaakov movement began (actually earlier, but that's not the point).
Currently (I live in Jerusalem and work with a couple of Haredi seminars there.) "knee-high" skirts are de-facto standard of the Ultra-Haredi women both in the Litvaks and Hassidishers communities:
When and by approval of what Rabbis, did knee-high skirts become the standard of Beyt Yaakov graduates and teachers? In other words, when Haredi women were allowed to show their calves (with stockings of course)?
(I'm not arguing about how long exactly - just below, long below etc.)