I echo Girlfriends, but even more would like to see an adaptation of Akiyama Haru's Octave.
If it has to be a schoolgirl setting, apart from Girlfriends, Yuhuko Suzuki's Candy might be nice. In the "somewhat more lewd" arena Hanjuku Joshi has already been mentioned, but Spring, Summer, Fall, Winter by Eiki Eiki could be good too.
Then again, I would probably already be happy if they ever managed to complete the missing pieces of the Sasameki Koto anime (or Aoi Hana for that matter).