I seriously doubt those are really uniforms. Unless you verified that every picture was from an actual school, and considering the plethora of schoolgirl fetishism, probably most of the pictures are sexually intended.
I Googled "Japanese high school uniforms"
Caption: " As mentioned, girls' uniforms in high school come in an array of fashions."
source
And here's an explanation for some of the short skirts we see.
Caption: "
High school students
Most schools require shool uniforms: generally a summer uniform, winter uniform, and PE uniform. The summer uniform usually consists of a short-sleeved shirt ("cutter shirt") and slacks for boys, and a pleated skirt and short-sleeved blouse for girls (often with the traditional sailor suit collar, but not always).
Most schools have strict regulations on how the uniform can be worn, which leads to many Japanese schoolgirls throwing on makeup, putting on their loose socks, and hiking up their skirts once they pass out of their teachers' line of sight.
source
So, this is
one reason for girls appearing in uniforms with short skirts.
And it's obviously a problem in some locals.
Caption and article:
Schoolgirls Told Skirts Too Short
Guidance counselors at a high school in Niigata, Japan decided that school uniform skirt lengths were getting too short, so the advisors took it upon themselves to design and publish dress-code posters and distribute the posters to all the high schools in their district.
It seems the poster and hemline advice is the first time such a public campaign has been made. Many schools in Japan, from elementary through high school, require students to wear uniforms. Hair length and accessories rules are also enforced.
News outlets learned of the Niigata effort, and reporters were sent onto the streets to grab high school girls and measure the distance of their hemlines from their kneecaps.
From the reports, we learned the secret to instantly raising and lowering the hemline, a school principal revealed what was observed by a railway employee at a train station, and the average distance of school skirts from knee-to-hem for major cities in Japan has been determined.
As random skirt-checks continue at the schools, parents have been sent requests to cooperate in the proper attire campaign. Girls oppose the efforts. One high school girl said "Short skirts are cute". A 16-year-old high school student walking in front of Niigata Station said about the campaign, "It can't be helped at the moment. It makes little sense. We wear short skirts because everybody does, rather than because they look cute."
source
YET we do see short skirts in some schools:
source
Plus the photo in the OP and this one,
were clearly taken at some school function.
My conclusion is that Japanese uniforms with short skirts are allowed in some schools, and where they're not the girls shorten them when out of school.
But back to the panties down around the girls' legs, I still await a good explanation.