They were called "loose socks" and came into fashion about 15 years ago. It was one of the only parts of a school uniform that wasn't strictly regulated so girls could "express themselves" with their socks.
Sometimes the socks themselves were over a meter long and were held up with "loose socks glue" for the perfect look.
And I worked in a Japanese high school in the mid-90s, and that's why I know, if you are curious.