About long skirts - pretty sure it was part of standing against sexual revolution movement, in particular against sexualization of women, like wearing mini skirts, by wearing very long skirts. As it opposed societal norms, it was usually adapted by counterculture types, like deliquents. If I remember correctly, something similar was also at the core of bosozoku girls movement, but I might be mixing stuff up.
As to why would a school had max skirt length - because having a uniform is all about, well, uniformity of students' look. The same reason they eq. require specific colors for elements like scarves. It doesn't need to have any deeper meaning really.