Kevin Brennan, the children's minister, said all secondary school pupils under the age of 16 should be prevented from leaving the premises at lunchtime so they are forced to eat healthier food.  The plan comes as new research reveals 80 per cent of children snub the school canteen for takeaways, sweets and fizzy drinks. It showed that some pupils left school to buy junk food more than 11 times a week.  Mr Brennan said locking school gates would tackle the soaring rate of childhood obesity, which currently stands at 17 per cent.  He said: "Some schools have a stay-on-site policy for 11- to 16-year-olds but lets the sixth form go off-site. I'm very strongly supportive of that approach.  "I would like to see more schools operating some sort of stay-on-site policy because its advantages are shown not just in improved uptake [of healthy school lunches], but also improved behaviour and community relationships."  Headteachers and parenting organisations welcomed a healthy eating initiative, but dismissed Mr Brennan's plan as "totally unworkable".  John Dunford, general-secretary of the Association of School and College Leaders, said: "Much as schools would like to keep children on site at lunch time, the number of exits - as many as 20 - make this almost impossible."  Andy Hibberd, co-founder of the Parent Organisation, a support group, said: "It is very 'nanny state-ist', totally unworkable and impractical. It is taking away parental choice and treating older children as babies."  However, Dr Colin Waine, chair of the National Obesity Forum, told the Daily Telegraph "something urgently needs to be done to tackle the obesity epidemic and stop children dying young.  "I don't like the idea of locking children in, but if it gets them back to having a healthy lunches that would be great.  "Near where I live I see most kids going straight from the school gates to the chippy, it's a huge contributory factor to soaring rates of childhood obesity.  "Over 17 per cent of British children are obese. These children are already showing risk factors for coronary heart disease, strokes, cardiovascular disease and type II diabetes, which can have devastating effects."  Ground-breaking research by academics at London Metropolitan University mapped the movements of hundreds of pupils over a week.  :: Children's top ten purchases included fizzy drinks, chocolate and fried chicken, while fruit and vegetables came 22nd out of 26 food types.  :: Girl's eat more unhealthly food than boys.  :: Only 6 per cent of pupils ate hot lunches, fewer than half of those entitled to free school meals.  :: One chip chop sold 63 portions of chips within half-an-hour of the end of the school day.  Jamie Oliver, the celebrity chef who first exposed the appalling state of school food in 2005, said that while meals have got healthy more effort needs to be made to encourage children to eat them.  "If you look at what's going on in schools where the catering staff have got the right support and where a 'dining culture' is developing, that's where it's working," he said. "But there's a big divide between these schools and the many where there are still problems."
Children could be locked in school grounds over lunchtime to prevent them buying chips and fizzy drinks, the government has signalled.