Posters aim to explain the nutrient changes to pupils
All primary school children in two areas of England can now have free lunches under a £40m, two-year trial.
It involves pupils in County Durham and Newham, in London, to see if behaviour, health and academic standards improve.
In Wolverhampton the existing means-tested entitlement is being extended. The trial costs are being split between the authorities and the government.
Meanwhile new nutrient-based standards for school lunches come into effect for England's secondary schools.
They were made obligatory a year ago in primary schools that have a meals service.
The standards apply to an average lunch rather than every meal, because of the choice on offer in secondary schools.
the maximum and minimum amount of energy in the food
the maximum amount of fat, saturated fat, non-milk extrinsic (NME) sugars and salt
the minimum carbohydrate, protein, fibre, vitamin A, vitamin C, folate, calcium, iron and zinc.
The School Food Trust, charged by the government with improving school meals, says the new standards are important because for many pupils school lunch is the main meal of the day.
Children's Secretary Ed Balls said: "I encourage all families who are entitled to a free school meal to claim this valuable support.
"For those who are not eligible, school meals are still a great deal and give parents good value for money.
"We know from recent research by the School Food Trust that parents with two children could save over £1,000 in a school year if they chose school meals - as well as the reassurance that their children are eating good quality, healthy food."
A survey by the trust has estimated that a fifth of low-income families in England are not even checking to see if they could claim free meals.
The 2009 annual school census showed a rise in free school meal pupils from 15.5% to 15.9% in primary schools and from 13.1% to 13.4% in secondary.
Schools minister Diana Johnson welcomes the scheme
It has been estimated that it would cost £1bn to make free meals universally available in England, as education unions would like to see.
In Scotland, all children in the first three years of primary school are to be entitled to free meals from August 2010, following pilots in five areas.
A sample menu complying with the new nutrient standards produced by caterers in Newham includes such things as chicken and vegetable pasta bake, beef chilli con carne and breaded fish.
Vegetarian options include spicy fajitas and roast vegetable potato pie.
Desserts featured assorted cakes, fresh fruit bags, yoghurts, cheese and biscuits.
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