It is, at first glance, every teacher’s nightmare. A dozen primary school children are playing in mud, and their feet and hands are caked in the kind of muck that would not look out of place on the floor of a farmyard.

First they jump in the stuff, digging their heels in so that it spills over their ankles. And now, horror of horrors, they roll it into dirty great dumplings which they lob to each other like soldiers passing sandbags down the line. Except that, as often as not, the dumplings miss their target, so that their contents, which seem to include large amounts of filthy brown straw, splatters over clothes and faces, to the delight of the throwers.

Not to worry, though. For as anyone familiar with vernacular building techniques will have spotted, these children are helping to construct a school extension. The school is Padstow primary in Cornwall and the mud-slinging session took place last summer. The extension, now finished, is home to a bright new music room.

It was built using cob, the traditional mixture of clay, sand and straw that is arguably the most natural and recyclable of all building materials.

For not only can the chief ingredients be dug from the very soil on which the walls stand, but when they are no longer needed, they can simply be returned to that soil, ready to be used again.

Not that the Padstow music room hasn’t been built to last. For once it has been compressed and dried by summer breezes, cob, with its undulating surface and breakfast-cereal texture, is hard and strong - durable enough to make a two-storey house.

Adam Weismann and Katy Bryce, who supervised the Padstow project, discovered cob in Oregon. The couple - he was born in Iowa, she in Welwyn Garden City, Hertfordshire - met in London in 1999 when Adam was studying to be a documentary film-maker. After several months, they married and set off for the West Coast of the United States to realise their dream of building a house together.

To find out about natural construction techniques, they signed up for a six-month apprenticeship with an outfit called the Cob Cottage Company.

They learned how to build stone bases for cob walls and how to make roofs out of timber and thatch (earthen walls must always keep their heads and feet dry). After completing their studies, they took a nine-month work placement on a project at Manaccan on the Lizard peninsula in Cornwall.

“We had no intention of staying here,” says Adam Weismann, “but we fell in love with it and settled in.” And, realising there was a potential market for cob building in the West Country, the couple, still in their late twenties, set up their own company - Cob in Cornwall - a year and a half later.

Initially they worked mainly for private clients, on projects ranging from garden structures (cob is particularly good for outdoor bread ovens) to yoga studios and study extensions. They have even built a handful of houses from scratch.

Often working with architects, or with experts in other materials (the Padstow school extension includes a staffroom partly built out of straw bales), they have been employed by the Royal Horticultural Association to restore a garden folly and have attracted the attention of the Duchy of Cornwall and that ardent supporter of traditional architecture, the Prince of Wales. “All in all,” says Adam Weismann, “we have had our hands on more than 100 buildings.”

And in the past two years, Cob in Cornwall has been moving into education.

Their association with the award-winning firm of architects Arco2 took them to Padstow primary, where headteacher Phil Banks was determined that the new extension should make maximum use of environmentally friendly materials and techniques.

The curving wall, two feet thick, consumed some 30 tonnes of cob, and in the month it took to build it children of all ages got involved, helping to mix the raw materials and press them into place. As the Padstow wall hardens, Adam Weismann and Katy Bryce are now working with a group of eight Year 10 students at Helston community college on a cob bus shelter in the town. This will have a base of local granite, a frame made of local oak, a slate roof in the Cornish style, and walls made from local clay and straw.

The students are all doing a two-year vocational course in construction, and the project will not only give them valuable experience in traditional building techniques, but it will also provide a meeting place for other young people in the town.

Watching Jamie, Tom, Aaron and Becky trampling the mixture of wet earth, straw and aggregate (traditionally, villagers would simply leave the wet cob on the floor of a stock pen overnight so that their animals could mix it thoroughly, while adding a few natural ingredients of their own), Katy Bryce expounds the magical effects of mud. “It goes way beyond teaching kids about traditional building skills,” she says. “Cob teaches them about themselves and about life - about working in a team, working towards a common goal, completing something and taking ownership of it. Take a self-conscious group of teenagers and get mud over them and it breaks down the barriers.” (Well, Glastonbury isn’t that far away and we’ve all seen the mud-fest that breaks out there when the heavens open.) This autumn, Cob in Cornwall undertakes its most ambitious project yet: a primary school to be built on Duchy land in Newquay.

The school has been designed by the Penzance architects Rodda Lloyd Travers. “Andy Travers has always wanted to design a building using cob and knew we were the people to come to,” says Adam Weismann. “I think the Duchy was also pressing to use cob on this project. We were invited to Highgrove last February to meet Prince Charles, and he has been very supportive.”

The architects’ drawings for Newquay Tretherras school show a long, low building standing on an impressive stone plinth and topped by a generous pitched roof in a style that echoes the Arts and Crafts movement. And betwixt plinth and roof there extends a great sweep of cob wall that will take 500 tonnes of Cornish clay and several helping hands to construct.

“We plan to use it as an educational tool,” says Adam Weismann. “We will have open days and workshops, because so many people want to get hands-on experience of using this material. And we like the idea of the students who will actually be using the building having a hand in building it.”

Yes, those hands will get very dirty, and yes, there will be scenes that resemble every teacher’s nightmare. But the chances are that, for all those involved, going to school will never feel quite the same again.

Getting to know cob

* Cob is a combination of clay-rich subsoil such as can be found in most parts of the UK a foot or so beneath the surface, an aggregate such as sand or quarry dust, straw for binding, and water.

* Mixed either by foot or, for larger quantities, with a tractor or mechanical digger, these ingredients produce a sticky material that can be piled up with a fork and trimmed with a shovel or mattock to make walls of virtually any desired shape.

* In fine weather, cob walls can dry out in a matter of hours. After six months, they can either be rendered with a mixture of lime, sand and water, or simply coloured with a wash of lime and water containing plant pigments.

* A year after construction, a cob wall will be as hard as concrete, and fit to last for centuries. Most counties boast a legacy of earthen-walled houses, often disguised by thick layers of plaster and colour wash. In Devon, the house in which Sir Walter Raleigh was born at Hayes Barton, near East Budleigh, dates back to 1450. It has been rendered with cement but underneath the walls are made of unbaked earth.

* Cob is recyclable, non-toxic and breathable, and its extraction, production, construction and deconstruction require little energy, making it the perfect material for eco-friendly buildings. And as Adam Weismann and Katy Bryce’s work with children in Cornwall demonstrates, it is a democratic technology, enabling people of all ages and abilities to produce buildings with their own hands and for their own use.

Adam Weismann and Katy Bryce have written a book, Building With Cob: a step-by-step guide, which covers all aspects of cob construction, including renovating existing structures and dealing with building regulations and planning permission. Building With Cob is published by Green Books and costs pound;25