A manga about a high school student couple who don't get along in real life but are lovey dovey when they chat via text messages
Mel Kano maybe? I haven't read it, but your description is pretty close to what I know of it.


  Shinjou Shouryuu is a healthy 17-year-old high-school boy. Many of his kouhai wonder why such a great guy doesn't have a girlfriend. Ask him and he'll tell you that between school, work, and saving his kouhais from bullies doesn't leave him much time to go on dates. So he instead goes online to "F.I.L. GATES". There in the chat room his alter-ego, Kakeru, is infatuated with the girl called Hal. Hal sends a message reminding him that Sunday will be the six month anniversary of when they first met (online) and that she wants to meet him in person! And let's just say it's not your typical first date.