School food champion Prue Leith says poor families should cut children's pocket money to pay for healthy meals.
Ofsted reported a decline, following new healthy eating guidelines
The chef, who heads the School Food Trust, said it would be better than spending it on sweets and fizzy drinks.
She wants head teachers and parents to help get another million children in England eating school food each day, up from the present 3.2 million.
Ofsted reported recently that fewer meals were being served since Jamie Oliver's "healthy eating" campaign.
Reasons were said to be complex but included a lack of consultation with pupils and their parents, and poor marketing of the new menus to make them more appealing to children.
Ms Leith said: "We know that eating school food is good for the health, well-being and performance of our children."
She told BBC Radio Four's Today programme that TV chef Jamie Oliver had done a "great thing" in highlighting the importance of nutritious food, but said he had "really upset" catering staff.
She accepted that school dinners costing about £2 were a "hefty" outlay for some families, particularly those with several children, but said many schools could offer subsidies.
"The average pocket money a child gets is £8.40 and even in primary school, most children get a pound a day to spend on the way to school," she said.
"Wouldn't it be better if that money went on school dinners rather than chocolate bars and fizzy drinks?"
Schools Secretary Ed Balls said a good diet was the key to a healthy, balanced lifestyle both in school and out of school.
"We now have some of the most stringent school food regulations anywhere in the world and parents can be confident that if their child has school dinners they will be eating healthy balanced meals.
"We need everybody from heads to parents to play their part and that is why I am supporting this campaign."
The National Association of Head Teachers is among other organisations backing the campaign.
General secretary Mick Brookes said: "The old adage 'you are what you eat' has never been more pertinent with growing numbers of children becoming obese and suffering the toxic effects of an unbalanced diet."
The School Food Trust was set up in 2005 with £15m from the then Department for Education and Skills to promote youngsters' education and health by improving the quality of food in schools.