I've already said some stuff about this in the
Sei & Shiori thread, but I basically agree with
mantidor in thinking that Sei/Shiori and Sei/Shizuka is shoujo-ai, whilst the rest are just friends (for now).
Whilst I can see why some people think Yumi is in love with Sachiko, I don't think Sachiko feels the same way about Yumi right now. They're both so insecure, and they have a difficult time opening up their feelings to each other.
As for Yoshino/Rei, I see them wanting to be each other's center of attention, particularly with Yoshino. That isn't to say they don't love each other because clearly they do, but I just don't really see it as a "romantic" love.