The Local Authority Catering Association says it has been getting requests to put sweets and fatty snacks back on the menu, particularly for morning breaks.
The association - which has 700 members across the UK - said many of the requests are from some of England's new academy schools, which do not have to follow the healthy eating guidelines laid down by the government.
Linda Mitchell, from the association, told the BBC: "Our members are telling us that they have been approached by academies to relax the rules and as providers to hundreds of thousands of schools we are concerned.
"They are being asked to put confectionery and other snacks back, especially at mid-morning. It is the return of the sausage roll to schools."
She said caterers were mainly being asked to fill vending machines with snack foods:
"It's a very small step before you are seeing the introduction of confectionery and fizzy drinks back in to those machines."
Jamie Oliver led a campaign for healthy school lunches six years ago, after which schools in England were required by law to meet stricter nutritional guidelines.
Sweets, crisps and fizzy drinks were strictly off the menu, and vending machines were either removed, or only filled with healthy snacks.
What do you think about this?
Should schools provide sweets and unhealthy snacks?
Is it up to parents and children to choose what they want to eat?