Washington ? Juvenile obesity may begin at home, but thousands of U.S. schools have signed contracts that feed the growing problem.

Over half of all high schools and junior high schools nationwide have struck deals with soft-drink companies or vendors, giving them exclusive marketing rights to their students, according to the Institute of Medicine, a health policy adviser to Congress. In exchange, the schools often get five- or six-figure payments that cover benefits their budgets don’t, such as SAT test fees for low-income students, new scoreboards, uniforms and even proms.

The problem is that the marketing deals often promote consumption of foods that are a nutritionist’s nightmare. Pennsylvania school food-service directors, for example, reported in June that their cafeterias’ best sellers were, in rank order: pizza, hamburgers and sandwiches; cookies, crackers, cakes and pastries; french fries; potato chips and cheese puffs; and sodas and sugary sports drinks.

Since, according to the institute’s study, 40 percent of kids’ daily food intake occurs while they’re at school, schools are implicated in a worrisome trend: Childhood obesity has doubled among teens and tripled for kids ages 6 to 11 since the 1970s. That’s according to the Institute of Medicine’s study, “Preventing Childhood Obesity,” which was released Sept. 30.

That’s no surprise to Diana Frey, a junior at Montgomery-Blair High School in Silver Spring, Md. “A lot of kids don’t even have time for breakfast, then they eat a crappy lunch here,” she said. “That’s a good eight hours without eating anything healthy at all.”

School meals served as part of the National Lunch and Breakfast programs aren’t the problem. Schools must provide specified amounts of vitamins and nutrients to be reimbursed by the Department of Agriculture under these programs. Rather, it’s food sold in vending machines and lunchtime a la carte lines that often has no nutritional standard and is intended to be popular — and profitable.

Montgomery-Blair is in a bind, Principal Phillip Gainous said. It signed an exclusive marketing deal with Pepsi in 1997, which currently pays the school $55,000 a year. The contract comes up for renewal soon, however, and the Montgomery County school system recently issued guidelines aimed at improving nutrition. Out went the soft drinks in Montgomery-Blair’s vending machines. In came fruit juices, chocolate milk and water. Pepsi’s vending-machine revenue has plummeted, according to Gainous. Pepsi may not renew the deal.

“The money is absolutely vital to the operation of the school,” said Gainous, who’s used it to buy computers and subsidize fees for SAT, achievement and Advanced Placement tests for low-income students.

“How are my kids from poorer areas supposed to compete with affluent kids without this money? If I push them to take these tests to get into college,” he said, “I have to be prepared to pay for them.”