Schoolchildren use the journey to and from school to fill up on "a secret mountain of junk food", according to a new survey.
School dinners are set for a major revamp from next month
More than half of secondary school pupils buy treats equivalent to 20 blocks of butter and 11 bags of sugar during the course of a year, it says.
There are estimated to be two million overweight and 700,000 obese children in the UK.
The research came from the School Food Trust, set up to improve school meals.
Nearly one in three primary school children buys fizzy drinks, crisps and chocolate while either on the way to or going home from school, according to the survey.
Ministers have announced a £220m package of reforms, including extra money, to improve kitchens and provide better ingredients.
Rules being introduced next month will require schools to ban all confectionery, such as chocolate, unhealthy savoury snacks like crisps, and fizzy drinks.
Manufactured meat products would be restricted and would have to meet the standards for minimum meat content, the School Food Trust said.
Under the new rules, meals will have to include at least two portions of fruit and vegetables per day, as well as oily fish at least once every three weeks and healthier drinks like fruit juice and semi-skimmed milk.
The School Food Trust is a new non-departmental public body established by the Department for Education and Skills (DfES) with £15m of funding in September 2005.
Its remit is to transform school food and food skills and promote the education and health of children and young people by improving the quality of food in schools.
The trust's chief executive, Judy Hargadon, said: "Parents always want what is best for their children.
"But this survey shows, despite their good intentions, children are consuming a secret mountain of junk food on the way to and from school.
"Too often this is happening because the meal they are getting at school is simply not filling them up and the hungry child is topping up with sugary and fatty foods.
"The introduction of higher-quality school meals starting in September will mean parents can feel confident that their child can eat better and do better."
The British Medical Association estimates that by 2020 one in five boys and one in three girls will be obese.
Researchers BMRB questioned 412 parents with children in primary or secondary schools for the SFT's survey.