|Infographic by The National Building Museum in Washington DC, USA.|
Obviously, activity is the key: If kids participate in movement-based after school activities, this might very well help to alleviate some of the affects of being driven to and from school. But then there's the whole petroleum consumption and its maladies.
I have noticed a related correlation. The schools that have trails (some safe, off-road route) that lead to them, have a much greater rate of children who walk or ride their bicycles. Of course this makes sense from numerous standpoints. First, the trails are safer than kids walking & riding on busy (or any) roads due to a decreased risk of auto/pedestrian accidents. Secondly, for those people who are paranoid about someone kidnapping their child ("The Ransom of Red Chief" is more realistic than this fear, but that's another story.) then trails are also much safer. It is much more likely that the (ubiquitous) kidnapper will utilize the services of an automobile when attempting a kidnapping. I would assume (having never seen the upside of kidnapping) that it would be much easier to pull off such a stunt from a motor vehicle than on foot or (nearly impossible) on bike.
Anyway, I think the above chart allows the statistics to speak for themselves. Active kids are healthy kids.