The girl on the left, Fujioka Shizuka, is not a school girl. She's 21! According to several webpages she's acting and modeling, especially for expensive brands which are selling school uniforms. I didn't know anything about her before her becoming "cute ambassador". Same for the one in the middle. I guess that her style falls into a category of harajuku fashion which is called "spank". (Correct me if I'm mistaken, I don't know much about this style) For sure she's not the single one in Japan dressed like this. I know the Lolita girl, though. She's in every edition of the Gothic Lolita Bible.
According to the page of the ministry of foreign affairs of Japan the girls had been elected because they are playing leading roles in their own fashion sectors: Shizuka-school uniform fashion,Yu-Spank(?) fashion, Misako-Lolita fashion. Their looks weren't important at all. Japanese cuteness isn't about looks, at least not about looks as in our Western sense of the word. When talking about the Japanese sense of cuteness I got the following for an answer: "Not everybody can be beautiful, but everybody can be cute." I suppose that the Japanese sense of cuteness is a very diplomatic thing. ;)