Apparently, the no-bike policy is no longer in effect. The Charlotte school district web site has a link to its curriculum, which includes bicycle safety in the primary grades, and their yearly report for 2007 includes a passage about middle school students who ride to school not using helmets 69% of the time.
(Yes, I was that bored.)
By the way, I teach middle school myself, and when I ask my students why so few of them ride bikes to school, they usually respond that bikes just aren't that cool. When pressed for specifics, they usually narrow it down to helmets. They think the helmets make them look stupid, and some of the kids don't want to mess up their hair. If they didn't have to wear helmets, they'd be more inclined to ride to school once in a while.