School tuck shops and canteens will come under the scrutiny of inspectors with a new brief to promote healthy living.
The Office for Standards in Education will encourage inspectors to evaluate whether pupils are adopting healthy lifestyles.
The changes, introduced to comply with the Children Bill, could see schools criticised by the watchdog for providing a poor diet of fatty, salty or sugary food.
Headteachers expressed concerns over the move to place another burden on schools during inspections and questioned whether schools had the main responsibility for children's diets.
Robert Dring, head at William Lovell Church of England secondary in Lincolnshire, has banned fizzy drinks from his school and limited the number of times chips are served. But he cast doubt on Ofsted's qualifications to inspect lifestyles, adding that they began his school's last inspection by tasting the cakes in the canteen.
He said: "If you start forcing people to eat salad at lunchtime, all they are going to do is bring in packed lunches full of all sorts of stuff that parents have purchased from big supermarkets.
"The issue is, how far are schools going to be held responsible for what children eat? I think the Government is picking a soft target. If it really wants to tackle obesity it needs to take on Asda, Tesco and McDonald's."
Margaret Morrissey, a lay inspector, said she and her colleagues were once given a different cake baked by a head's mother for every day of the inspection. She said: "We don't binge on the chocolates like teachers do during inspections. But when you're stressed you do comfort-eat."