Pupils in England are being presented with healthier choices on the school dinner menu and in vending machines.
It is hoped the changes will improve behaviour and achievement
The Department for Education guidelines mean meals must include at least two portions of fruit and vegetables and deep-fried foods are restricted.
The government has earmarked an extra £240m to subsidise healthy ingredients until 2011 and school cooks will receive extra training.
From 2008, secondary school pupils will also be offered cookery lessons.
The overall standards aim to improve pupils' health, behaviour and concentration.
The guidelines are based on recommendations by the School Meal Review Panel and follow a campaign by TV chef Jamie Oliver to improve the quality of school dinners.
Other key measures include:
- Not less than two servings per day of fruit and vegetables
- Oily fish should be served at least once every three weeks
- Bread should be available every day
- Free, fresh drinking water should be available
- Salt should not be available at lunch and ketchup and mayonnaise should only be available in sachets
- No more than two portions of deep-fried foods in a single week
- Manufactured meat products such as chicken nuggets may only be served occasionally and only providing they meet minimum standards for meat content
- The only savoury snacks available at lunchtime should be nuts and seeds with no added salt, fat or sugar.
The number of obese or overweight children in Britain aged between two and 15 has risen to about 30% in recent years.
The School Food Trust was set up in 2005 with £15m of funding from the DfES to promote the education and health of children and young people by improving the quality of food in schools.
Trust chief executive Judy Hargadon said children who had a "better" diet reaped health and social benefits.
Education Secretary Alan Johnson said the new guidelines would "make a difference in terms of healthy lifestyles and child obesity for a start".
He told BBC Radio 4's Today: "Teachers have been telling us for a long time that if I child comes into school having had little or no breakfast and something deep-fried for lunch they are not going to concentrate, they are not going to be learning properly."
He said that offering cookery lessons was part of a drive for pupils to be able to leave school with practical skills.
The voluntary course will comprise 24 one-hour lessons - at the end of which pupils will receive a certificate.
Mr Johnson said the lessons could be offered by staff other than teachers and out-of-school hours.
A Soil Association report suggests spending on ingredients by primary schools has increased from an average of 47p per pupil per day in 2005 to 51p this year.
But some are still spending as little as 41p per child per meal, according to its survey of 74 local authorities in England.
Compass, a catering company which supplies nearly 2,000 schools, colleges and universities welcomed the extra cash.
UK chief executive Ian el Mokadem said: "Extra funding is key. One of the problems was that if you're trying to cook a good meal in a primary school for 37p that's pretty tough," he said.
The Automatic Vending Association said the new guidelines were misguided and the take-up of school lunches was likely to decrease because of an increase in cost..
Chef Anthony Worrall Thompson said he felt more stringent measures were needed - and more control over the food children were bringing into schools..
He said: "I think if the government are going to tackle this they can't just scratch the surface. I would like it to be one set meal on offer - no packed lunches - and maybe we need to make that free.
"I'm sure the government would baulk at that but there are savings long-term to be made in terms of the health benefits."
He added: "I do think we've got to take these stringent steps earlier - and tell kids what to do. Don't let them have a choice."
The Scottish Executive is also planning to ban junk food from schools and to create a law encouraging more pupils to eat school meals.
In Wales, a working group on nutritional standards published a report calling for junk food to be restricted on school menus.
The report, Appetite for Life, recommends a ban on the sale of sweets and crisps and an increase in healthy food on the menu.
Northern Ireland's education department has put out proposals for public consultation.