Teenagers who opt for salad instead of burgers and chips have told experts they are ridiculed and even attacked for their efforts to eat healthily.

A new study to be published this week reveals how pupils say going for healthy food marked them out for stares and peer-group taunting, adding that it gave bullies a "licence to pick on you".

Its findings reveal how some teenagers try to hide the fact that they are ditching chocolate snacks for apples or, in extreme cases, avoid school lunch halls completely rather than face the salad bar bullies. Overweight teenagers are the prime targets, with jibes centring on their attempts to slim down by avoiding fatty food, researchers learned.

The findings are part of a series of papers that will be discussed at a conference of leading sociologists and health experts beginning in London on Thursday and include a warning that the government's Healthy School Programme has inadvertently helped to marginalise the country's rising number of obese children.

National statistics show that more than a quarter of all girls and 20% of boys in England are overweight, with the numbers of critically obese children rising each year.

British education minister Alan Johnson has responded by pledging �500 million for healthier school meals and embracing initiatives to convince teenagers to improve their diets. Schools have been challenged to reach healthy school status by 2009, effectively a government target to ensure that children are eating healthily.

The intervention followed celebrity chef Jamie Oliver's campaign for a better service but the children encouraged to turn their backs on Turkey Twizzlers are now facing up to the bullies.

Dr Penny Curtis of Sheffield University, author of the report, said children felt uncomfortable when they ate healthy food in front of other pupils.

Her report said: "For young people with obesity, apples can be dangerous things."

Curtis said: "I think it really is a case of unintended circumstances. The trouble with the healthy eating agenda is that it is adult-led and nobody has sat down with the children and asked them what it actually means to them. It's not enough just to put apples in the canteen. There has always been bullying; children who are overweight have always been bullied but there has been such a focus on healthy eating that this has come up.

There are a lot of reasons for it but there needs to be a cultural change for things to be different."

Louise Burfitt-Dons, founder of Act Against Bullying, said: "Children feel under pressure if they are trying to conform to eating patterns at home. For example, if a family eats organic food at home, it can be hard to follow that through at school. Tribalism is on the increase and children that are different in any way will be targeted."

National Union of Teachers general secretary Steve Sinnott said: "While the government has issued some good advice on targeting prejudicedriven bullying, it should be given a much higher profile. All schools should be funded sufficiently to provide trained counsellors to help staff follow up incidents of bullying."