I went to a girls` high school in Japan for a stint... And it`s normal. It`s nothing sexual.
It`s just a difference in cultural norms. Thinking about the different attitudes toward certain behavior in terms of your own culture is sort of like trying to fit a square peg into a round hole. It`s just not going to work.
But I do have to say - I didn`t really find the normal behavior of Japanese girls all that different from that of those in the US... The only big difference was that in Japan, no one thought anything of it so it was totally normal to behave that way even with guys around. In the US - it was totally normal for girls (yes, high school age) to have sleep overs including bathing together, and "bra parties". It was just not something that was talked about when guys were around because, well, there was this horrible fear that they`d think you were a lesbian when you weren`t. And no girl wanted the guy she had a crush on to think that she wasn`t available.
ETA;
If you think that hiking skirts up is something unique to Japan, I invite you to visit ANY private US school that has students wear a uniform. It usually starts in the last couple years of grade school, and 100% by middle school. (If you don`t, you`re generally a social outcast.)