Thanks to Green Machines, school snacks needn't be junk food.

In Dagenham teenage school-children, notorious for skipping breakfast and eating badly, can start the day with an organic yogurt and a muesli bar, have fresh, squeezed orange juice at break and a bag of organic crisps to supplement their lunch.

These relatively virtuous snacks are part ofmoves to put into practice the healthy-eating message directed at schools. However, for all the talk of encouraging children to eat better, visit almost any school and you'll be appalled by what the children eat. Not just in their canteens or the chippy at the corner, but from vending machines in the corridors. These represent business worth about £1bn in schools, dispensing fizzy drinks, chocolate and crisps and coining a tidy profit that's essential to schools' budgets. In some cases the machines actually subsidise the catering.

Obesity among children is at alarming levels, yet health education is constantly undermined. Whatever good intentions children may have about eating well, they're easily swayed by adverts and brands, and if there's no choice they'll give in.

But the Green Machine offers one. The scheme was dreamed up by David Berney, who used to run his own restaurant. He is an advocate of organic food and wants to make it more accessible to children. The scheme is backed by Yeo Valley, Britain's largest organic dairy brand, and Jordans, the Bedfordshire-based family-run grower and maker of cereals, who farm traditionally and without pesticides. Their yogurts and muesli bars are in the machine alongside orange juice, mineral water and Jonathan crisps, all of which are sold for less than the usual price.

Organic doesn't always mean nutritionally worthwhile. But Berney insists that products have an ethical value, too. One, the Dubble chocolate bar, is fairly-traded milk chocolate from a company partly owned by the Ghanaian cocoa-producers.

Another criterion is that there should be no additives. Berney approached the Hyperactive Children's Support Group for advice, and the contents of the machines are monitored by them. The group works by addressing what hyperactive children eat, and improving their diet. "I'm horrified by what's sold in some schools' tuck-shops and vending machines. We saw the machines as a leap forward, an option for schools," says Mick Giovannelli, the project director for the HCSG's London Centre. "Our research shows that hyperactive children are chemically sensitive. Better-quality snacks for children in schools can only be good news.'

Melissa Wright, the healthy schools co-ordinator in Barking and Havering in east London, also saw the machine as "an excellent way of shifting the balance, so children can think about what they're eating." There are two in her area and she's encouraging more schools to install them.

All Saints Catholic School in Dagenham was one of the first to pilot a machine. It has been there since September, and though the novelty has worn off, it still has to be refilled every day. "By 9am the yogurts have gone," says the head, Desmond Smith. To him it represents more for his 1,200 pupils than the sum of its robotic parts: "It fits in with a more holistic approach to making healthy choices in their lives." He'd also like it to relate to the curriculum: in science, learning about nutrition, agriculture and the environment; and in ethics, in issues about world trade.

For kids spending the 40p cost of most of the contents, their choice is, so far, more likely to be determined by taste than global concerns. Francesca Anastasia, 11, likes the crisps. "I sometimes have a Frusli bar and an orange juice. My friend Jenna buys orange juice and a Dubble."

Berney aims to have 400 machines in schools by the end of the year, and 4,000 within four years, but he has a wider vision. The participating food companies also fund an educational element. They're recruiting a writer to work with All Saints pupils on a play with an environmental message, and a fashion graduate to design costumes. The play could be put on at other schools with Green Machines. There are also plans for school trips to Yeo Valley and Jordans farms.

Schools are a battleground where food manufacturers vie for custom. The Green Machine is spearheading the arrival of more ethically and nutritionally aware forces.