Absences down after chef changed junk food menu - a result which is a boost for celebrity as he struggles for US support

He has been ridiculed by the chat show host David Letterman, accused of high-handedness by a local radio DJ and reduced to tears by recalcitrant fast food-consumers during his war on American obesity. He has even dressed up as a giant pea pod in an attempt to turn the US on to his healthy eating agenda.

So Jamie Oliver will doubtless be relieved to hear of a timely reminder of his more gilded reputation back home. Today an audience of prestigious economists was told that the healthier school dinners introduced by the celebrity chef had not only significantly improved pupils' test results, but also cut the number of days they were off sick. The effects, researchers said, were comparable in magnitude to those seen after the introduction of the literacy hour in the 90s.

The proportion of 11-year-olds in Greenwich, south London, who did well in English and science rose after Oliver swept "turkey twizzlers" and chicken dinosaurs off canteen menus in favour of creamy coconut fish and Mexican bean wraps, according to a study of results in the south east London borough.

The number of "authorised absences" — which are generally due to illness – fell by 15% in the wake of his 2004 Feed Me Better campaign, brought into the nation's sitting rooms via the Channel 4 series Jamie's School Dinners.

But the annual conference of the Royal Economic Society also heard that the poorest pupils – those who are eligible for free school meals – did not seem to benefit. Instead it was mainly children from more middle class homes who saw their scores boosted after Oliver's junk food ban was implemented.

The researchers estimated that the proportion of students who got level 4 in their English Sats at key stage 2 increased by 4.5 percentage points after his intervention.

The percentage who got level 5 in science was up 6 percentage points, they reported.

Oliver described the research results as "fantastic". "It's the first time a proper study has been done into the positive effects of the campaign and it strongly suggests we were right all along," he said.

"Even while doing the programme, we could see the benefits to children's health and teachers. We could see that asthmatic kids weren't having to use the school inhalers so often, for example.

"We could see that it made them calmer and therefore able to learn."

The chef said it was further evidence that faster movement was needed towards improving take-up of nutritious, home-cooked school meals across the country, by training dinner ladies, getting kitchens and dining halls up to scratch and educating children and parents.

The presentation of the findings comes at a convenient time for Oliver, whose US version of the Greenwich project, currently being shown on the ABC network, has seen locals in America's unhealthiest city, Huntington, West Virginia, give him short shrift.

"We don't want to sit around and eat lettuce all day," radio DJ Rod Willis snapped at Oliver during the first episode of Jamie Oliver's Food Revolution. "You come to town and you say you're going to change our menus. I just don't think you should come here and tell us what to do."

Last week the Essex-born chef appeared on the Late Show, and was forced to listen to host David Letterman predict he would fail in his crusade to transform people's health. Letterman insisted diet pills were the only way to lose weight in the US.

Michèle Belot, of Oxford university's Nuffield College, and Jonathan James from the University of Essex, monitored results and absences in five neighbouring local authorities – chosen for their socio-economic similarities to Greenwich — as a control. They looked at figures from 2002 to 2007 – skipping the school year 2004/5, when the new menus were introduced.

The effects seen, they said, were particularly impressive given that they emerged within a relatively short period of time, and that the campaign was not even directly targeted at improving educational outcomes.

"As indicated by the relative fall in absenteeism, it is likely that children's health improved as well, which could have long-lasting consequences for the children involved not only through improvement in educational achievements, but also in terms of their life expectancy, quality of life and productive capacity on the labour market," the study said.

A survey by the Association of Teachers and Lecturers presented at its conference in Manchester today found that almost seven in 10 union members thought all primary school pupils should be given free school meals.

The same number wanted controls in place to limit the sale of chocolate, sweets, crisps and deep fried foods.

A third said the dining room at their school was unsuitable, and 56% said they had seen pupil behaviour deteriorate after eating food with a high fat or sugar content.

James said the research team was now looking at why children from poorer homes seem to miss out on the benefits of the changes brought in by Oliver.

"This is a source of concern, in particular in light of using school meals as a way of reducing disparities in diet across children," the report said.

It suggested the difference might be because those from richer backgrounds adjusted more easily to changes in school meals, or because the less privileged students were more represented among those getting lowest scores, and improvements were harder to achieve for those at the bottom than in the middle.

Meanwhile there are signs that the tide in the US may be turning in Oliver's favour just as it did in Greenwich, where initial hostility from dinner ladies eventually turned to adoration. More than 100,000 people have signed an online petition supporting his campaign for better school food. After he appeared with Oprah Winfrey on Friday, 7.5 million people tuned in to watch his show.