Wrong war. It was a global fashion trend to dress kids in militaristic outfits before, during and immediately after (perhaps inexplicably, given the horrors thereof) World War I. The sailor-type outfits were introduced into Japanese girl's schools at Fukuoka Jogakuin in 1921, and remained popular long after the style died out most everywhere else. And the tag you're looking for is "serafuku", which is included in the "school_uniform" category.
A fact worth also mentioning is that the traditional boy's gakuran school uniform borrowed its style from Preussian Officer uniforms.
If you think about it, the fact that the girls were styled in the way of enlisted men while the boys got their inspiration from officer gear is probably not a coincidence either.
