Contract caterers have threatened to raise prices for the supply of school meals or pull out of the market completely if their profits continue to fall because of the Government's ban on junk food.
Only a few weeks into the school year, contractors are already warning that pupils - unable to get what they want from school vending machines - are buying their favourite snacks and drinks at newsagents and chip shops beyond the school gates.
Catering executives said that unless they received adequate compensation for abandoning junk food, they would quit schools where they could not make a decent return.
Ian El-Mokadem, the UK chief executive of Compass, told the Guardian: "As new standards apply, there is a need for us to go and renegotiate on a school-by-school basis.
"We won't sign up to operate in an environment where we don't think we can do a good job - provide proper quality food and a fair return for our shareholders. There will probably be a few schools where we say: ‘We can't work here any more. We can't make it work'."
Meanwhile, Sodexho is encouraging schools to take a more gradual approach to the introduction of healthy snacks.
"Some secondaries have implemented early and seen great losses," a spokeswoman told the Guardian. "We aim to look at what the loss could be and how we can mitigate it… As a last resort, if we can't make money, we would have to renegotiate."
At Rawmarsh comprehensive in South Yorkshire, where children are not allowed out at lunchtime, some parents are apparently so unimpressed with the new menus that they are passing contraband meals to pupils through the school railings.
Every lunchtime, a group of mothers are reportedly taking orders for more than 50 unhealthy meals, including burgers and fish and chips, from a graveyard next to the school.
By Daniel Thomas