Over two years later --- I re-read the series (one of my favorites) and came to some slightly different conclusions. Tsubaki going to his high school doesn't really make a difference in the peculiarity of their relationship. I don't want to sound like a snob, but she can't really understand him. Cultural class and socio-economic class are two very different things. Several people that I was friends with from my childhood could never understand my musical tastes, love of history, or literary tastes, and they had no desire to understand or appreciate them. I, in my own way, loved these people, but they had no interest in understanding me as I grew up. They expected me to deal with them in the way we were when we were ten years old. Fighting to get an academic standing to be with your friend is undoubtedly a good goal, but the greater goal would be trying to understand why he is the person he is (and this is an argument divorced from gender, as a number of cultured, intelligent girls I have known have gone through the same scenario). As a fan of this manga, I will always be a Kaori fan, loving Emi and Nagi as well (can anyone tell that I am a guy?)
Bottom line:
I'm no a snob, but I think that coupling with Tsubaki would destroy Kousei's career. She has no idea in hell where he is coming from. She has known him for years and has yet to try and find out what motivates him. When I first read this manga, I thought that Kaori was the best person for him (pushing him forward over and over again). After she died, I thought that Nagi was the best person for him, someone fighting tooth and nail for the same goal as him. Tsubaki could be the person for him, if only we had ever had even a hint that she had even the slightest understanding of who he really was and what he was striving for.