TV chef Jamie Oliver's campaign for healthy school dinners boosted pupils' test results, researchers say.
Primary pupils in Greenwich, London, who took part in the Feed Me Better scheme, got better results than those in neighbouring boroughs, they found.
The study by Oxford University and Essex University also found them less likely to be off sick from school.
The schools replaced junk food and processed dinners high in fat, salt and sugar, with healthy school lunches.
And the campaign, which ran between 2004-5, led to major changes to nutritional guidelines on school dinners and mass retraining of dinner ladies.
The researchers assessed the impact of the campaign by comparing pupils' scores in the national curriculum tests at the end of primary school between 2002 and 2007 with those of peers in similar boroughs.
They also looked at attendance records for the same period.
In Greenwich, the proportion of children reaching the required standard at the end of primary school rose by 4.5 percentage points in English.
In science, the proportion of children doing better than the expected level increased by up to 6 percentage points.
It also found that attendance rates rose 15% in the Greenwich schools.
The researchers said that by comparing the Greenwich results with those of neighbouring boroughs with similar levels of deprivation, where improvements were not so pronounced, it showed the improvements were down to the effects of the campaign.
And staff involved in the scheme said they felt there were improvements from early on.
Head teacher of one school involved, Kidbrooke School, said: "Because the children aren't being stuffed full of additives, they are much less hyper in the afternoons now."
But the researchers suggested the raised results could be due in part to a "placebo effect", as the pupils were aware that they were involved in a very high profile campaign.
However, Oliver said the results were "fantastic".
"Even while doing the programme, we could see the benefits to children's health and teachers," he said.
He added: "It's just yet another piece of evidence that we need to move faster in terms of improving take-up of nutritious, tasty home-cooked school meals across the country - training and supporting more dinner ladies, getting the kitchens and dining halls up to scratch, educating kids and parents about how easy a good diet can be."
Michele Belot, of the Nuffield College, Oxford University and Jonathan James of the Department for Economics, Essex University, will present their findings at the Royal Economic Society's annual conference this week.