PHILADELPHIA, July 29, 2005 — -- When a Philadelphia school district recently slimmed down its lunch offerings and banned sodas from vending machines, educators hoped the moves would help stem the tide of childhood obesity.
But as school officials continued to see an overweight student body, they began to suspect that the real culprit behind the children's weight problems was lurking beyond school walls.
A survey of 600 Philadelphia students found that more than 50 percent of them stop at corner stores on the walk to or from school, spending an average of $2 each day.
So the school district called in mapping experts from the nearby University of Pennsylvania. Using handheld devices, mapmakers from the university's Cartographic Modeling Laboratory plotted every eating opportunity around five inner-city schools.
"Their ability to pack on as many as 3, 4, 500 calories in the 15 minutes after school really was an eye-opener for us," said Gary Foster, clinical director of the university's Weight and Eating Disorders Program.
Sure enough, the school with the highest percentage of overweight children -- nearly half of the student body -- was surrounded by twice as many food opportunities as the school with the lowest percentage of overweight kids.
"If it's about where you live, this is not all your fault, and the implication is policy. The changes need to be made at the neighborhood level," said Amy Hillier, the lab's research director.
School officials find a challenge in providing a solution. While it's easier to regulate the foods kids are served at school, it is much more difficult to keep them away from the corner store. In fact, some nutritionists say it's impossible.
Instead, the schools have set up mock corner stores, teaching students how to make healthier choices. Students who put that knowledge to work are rewarded with school supplies and raffle tickets.
Nutritionists say if they can get kids to cut back by even 100 calories a day, it will make a significant difference.
ABC News' Nancy Weiner filed this report for "World News Tonight."