The challenge of childhood obesity is all about improving the kid's body composition. Body composition is generally expressed as a "percentage of body fat", and like golf, a high percentage is always bad, and a low percentage is always good.
There are three generally accepted ways to measure body composition. They include underwater weighing which is the most sophisticated, the most time consuming, the most expensive, and almost always performed in an exercise physiologist's lab. A second method, the skin fold technique, involves measuring the thickness of a participant's skin with a caliper. The resulting data is plugged into a formula which produces a percentage of body fat. The third, electronic impedance, is a computerized method that yields similar information.
None of the Accepted Methods are Being Used
All three have their issues, but interestingly enough, none of them are used in our nation's schools to measure kid's body composition. They're all too time consuming and too expensive. School systems lack the funds to do this kind if testing, so they settle for a method known as Body Mass Index, which is just the modern term used to describe the age old height weight charts that have been around since the dinosaur age.