I also went to a very diverse highschool (recently; class of 2012), and saw the same self-segregation. However, when I saw the census dotmap that was on here a while ago[1], it was obvious that even though the school district was diverse, each of the neighborhoods was clearly a "${race} neighborhood". I'm willing to believe that the self segregation in-school reflected the segregation outside of school; that more diverse schools and neighborhoods might solve this.

[1]: http://bmander.com/dotmap/index.html though it seems to be down now