I'm going to clear some of this up. I am a middle school of a school that feeds into both NC and Shadle. All of NC's middle schools feed into NC and another high school. Over the years I have been coaching at the middle school, I have noticed that my boys tend to go to NC while my girls go to Shadle (This is true at another nearby middle school too). There are some exceptions, but in general this is true.
I have coached more than half of the NC runners. I've known most of the NC (and Shadle) runners since Fa 2008. Since I have been coaching at the middle school (Sp 2009) I have only known one student that chose NC over the school in whose boundary they live in. There are a few that have moved after middle school, but as of eighth grade all but one has lived in NC boundaries. I know this because the coaches (both NC and Shadle) are so careful about who they talk to in middle school in fear of being charged with recruiting out of their boundaries. Part of my job is to find out who is going where so I can connect them with the high school coach.
This is only speaking of running. In other sports, NC loses a lot of athletes to other schools.