Change has been slow. Schools made money on sugary drinks, and some set up rogue drink machines that had to be hunted down. Deep fat fryers, favored by school administrators who did not want to lose popular items like French fries, were unplugged only after Wayne T. Grasela, the head of food services for the school district, stopped buying oil to fill them.

But the message seems to be getting through, even if acting on it is daunting. Josh Monserrat, an eighth grader at John Welsh Elementary, uses words like “carbs,” and “portion size.” He is part of a student group that promotes healthy eating. He has even dressed as an orange to try to get other children to eat better. Still, he struggles with his own weight. He is 5-foot-3 but weighed nearly 200 pounds at his last doctor’s visit.

“I was thinking, ‘Wow, I’m obese for my age,’ ” said Josh, who is 13. “I set a goal for myself to lose 50 pounds.”

Nationally, about 17 percent of children under 20 are obese, or about 12.5 million people, according to the Centers for Disease Control and Prevention. The C.D.C. defines childhood obesity as a body mass index at or above the 95th percentile in comparison to children of the same age and sex in the center's growth charts, a reference population based on children in the United States primarily from the 1960s and 70s.

Obese children are more likely to be obese as adults, creating a higher risk of heart disease and stroke. The American Cancer Society says that being overweight or obese is the culprit in one of seven cancer deaths. Diabetes in children is up by a fifth since 2000, according to federal data.

“I’m deeply worried about it,” said Francis S. Collins, the director of the National Institutes of Health, who added that obesity is “almost certain to result in a serious downturn in longevity based on the risks people are taking on.”