If the yeshiva world will value middos and tzniyus for its own sake instead of “frummer than thou” status, girls will respond and naturally wear more tzniyus items.
From my perspective, the “yeshiva world” (which admittedly has no precise definition) does value middos and tzniyus, and the girls in large part do dress and act more b’tznius.
Of course, there’s an element of “no true Scotsman” here, because tznius is very much part of the culture we call “yeshivish”.