If you noticed, most of the Japanese school uniforms are worn with knee socks. Its not really a tradition rather more of a fashion statement and it actually looked cute. In some of the knee socks in the pictures shown many are Ralph Lauren knee socks. RL is a known designer clothes brand that specializes in American sports wear and the Japanese people warmly embraced the American fashion trend and became a part of their lifestyle and adopted even in their cute and sexy school uniforms. There's even some lingerie designs for girls made to be worn under a school uniform, its a bit off the school uniform topic but the Japanese people made it popular by having magazines with sexy school girl in their lingerie and it looked nice.
Even in school sports activities, the Japanese people made their own stylish and sexy version of P.E. uniforms for girls. Cute and sexy Japanese school uniforms for girls are just nice.