Burgers are one of the foods campaigners want children to avoid
Schools and councils are being urged to make it harder for children to swap their school meal for a takeaway.
Rising levels of obesity are being fuelled by the ready availability of fast food, said the School Food Trust.
It wants schools to close their gates at lunchtime and councils to stop new fast food outlets opening nearby.
But the Local Government Association said it could not force schools to shut their gates and that food retailers could challenge licence refusals.
The trust has issued a "league table" of the local education authority areas with the most takeaway and sweet shops per secondary school.
Seaside towns - with dozens of outlets aimed at tourists - and inner city areas, fare the worst.
Topping the list is Brighton and Hove, with 46 per school, closely followed by Blackpool and Hull.
School Food Trust chief executive Judy Hargadon said: "At the moment school canteens have to compete with a myriad of take-aways, chippies, and sweet shops for pupils' dinner money.
"We all know that some children will go for chips five times a week if they are allowed to.
"The problem is that this is damaging their long-term health, and is also threatening the viability of school lunch services."
She is backed by a dietician from London's Great Ormond Street Hospital.
10 WORST LEA AREAS
1 Brighton and Hove 46.11 outlets per secondary school
2 Blackpool 40.63
3 Kingston upon Hull 40.00
4 Reading 39.17
5 Middlesbrough 38.33
6 Manchester 36.95
7 City of Bristol 36.94
8 Inner London 36.66
9 Newcastle upon Tyne 36.21
10 Gateshead 35.00
Source: School Food Trust
Paul Sacher said: "Children face daily temptation from junk foods and many find them hard to resist.
"These foods play a big part in weight gain, have little nutritional value and contribute to health problems later in life."
Some councils and schools are taking steps to try to restrict the ability of children to buy fast food during school hours.
Leicester City Council is drawing up plans to ban mobile burger vans from areas around schools, and some schools already operate policies which stop children from leaving at lunchtime.
General secretary of the Association of School and College Leaders John Dunford said there were practical issues that could prevent all schools and colleges keeping children on site at lunch time.
He added: "For instance, large schools with multiple entrances would struggle to provide adequate levels of supervision. Other schools were built with canteens that are physically too small to cater for every pupil."
And head of the National Union of Teachers Steve Sinnott said children's hearts and minds needed to be won in the battle against junk food.
He added: "Closing the school gates at lunchtime could simply mean that youngsters will flock to junk food outlets at the end of the school day."
Dr Sacher encouraged other local authorities and schools to take steps to protect child health by restricting the number of new licences issued for fast food outlets in the areas around schools.
However, the Local Government Association said this could be hard to enforce, and any refusal to grant a licence could be challenged.
A spokesman said: "Councils are up for using all the powers at their disposal, including planning, to play their important part in tackling this weighty issue.
"It is wrong to imply that planning laws that ban junk food outlets around parks and schools are a potential 'silver bullet'.
"Local authorities have to base their decisions on good evidence of potential harmful impacts, and the matters they are allowed to take into account by law."
She said that while education authorities could encourage schools to adopt "stay in" policies, they could not be compelled to do so.