The short answer to your question is, NONE. There is no symbolism, nor even a tradition of women wearing white gowns to graduate from high school.
Here's the long answer ---
As it turns out, girls in white is not any long-standing "tradition" but is more likely a recent innovation having more to do with style preferences of the day.
More the norm nowadays is this: Guys in one color, girls in another. Understand that the whole idea of high school graduation gowns is based on the traditional academic regalia worn by university
graduates. However, in high school there are no different colleges. One color alone signifies graduation. Early on, graduates from high school didn't even wear gowns. This picture from 1923 shows the graduation class just wearing dress clothes. Yes, the women are in white, but
only because white was considered "dressy" in those days.