The American School FoodService Association estimates that 13 per cent of the 86,000 public schools now sell fast food, up from 7 per cent in 1991.
McDonald's, Pizza Hut, Taco Bell and other fast-food chains and conglomerates including Coca-Cola and Pepsi, are delivering food to schools or even setting up full kitchens, in a bid to create brand loyalty among a huge market of young consumers.
Cash-strapped schools are so keen to turn a profit from the deals that they have resisted attempts at regulation.
Drinks companies have made deals for exclusive access to students. While federal law prohibits the sale of soft drinks in public school lunchrooms, vending machines have been installed in corridors under deals with Coke and Pepsi. Only Pepsi products are sold in schools in Jefferson County, Colorado, under a deal to help pay for a football stadium.
The Keller Independent School District near Fort Worth, Texas, was the subject of a bidding war between Coca-Cola and rival Dr Pepper. Coke won the exclusive contract in exchange for more than $4 million (Pounds 2.5m), muchof which is to be used to buycomputer equipment.
There are more than 43 million children in US public schools. They spend an estimated $15billion a year and influence $160bn spending by their parents.
Orie Brafman, for the Center for Commercial Free Public Education, said: "These companies realise that consumer choices and consumer behaviour start early in life. But this is bringing in junk food. They know these kids are a potential lifelong market."