Loose socks are a thing among Japanese school girls. This Wikipedia article explains them well. The idea is that you wear very long, very loose socks with your shortened school skirt. They have been around for ages but I hadn`t realised just how long they could be.
The picture above is of loose socks being sold in a mall near here. The longest ones there are 150 cm long! That is taller than a lot of the girls themselves. Either they are catering to a niche market of gargantuan Japanese schoolgirls (scary) or there are a lot of folds in them there loose socks.