Alphington primary school, in Exeter, has a new classroom. It's hexagonal, with round, glass windows, and the pupils are very proud of it – not surprising, as they built it themselves.
"We got really muddy," says Tony Hutchins, 10. "You have to get the mud really wet to build with it."
Ben Vanstone, 10, chimes in: "But we didn't just put the mud straight up, we had to weave it through the slats so it would stay."
The classroom is of wattle and daub with a turf roof, much like – as their head, Debbie Buckingham, reminds them – the lavatory block that their partner school in Uganda will be building with money that the Alphington pupils have raised recently.
Putting up an outdoor classroom like this, she says, means far more than simply adding an extra space for story-telling and outdoor activities. It offers children opportunities to learn about history, geography, science and art, and has given its junior builders a chance to gain practical skills, practice teamwork and just get gloriously muddy.
"We have a skills-based curriculum and something like this helps to bring it alive," she says, "Also, a lot of our children don't always get out much, so we try to get them outside with as many things as we can." The school is actively green and already has a greenhouse made from empty plastic drinks bottles, a thriving vegetable garden and a pond. During a recent Anglo-Saxon week, pupils camped in the school grounds, allowing the new mud classroom and its adjacent mud bread oven to come into their own.
Behind the project are two Devon artists who are passionately committed to working with cob, the mixture of sub-soil, straw and water that many old houses in the West Country are made from. Both fell into the work by living in cob houses and learning to repair them themselves, and from that has sprung a vibrant 10-year partnership. It has seen them create cob sculptures and buildings in numerous locations, as well as running workshops for adults and children, and travelling the world to talk about their work and to study construction methods in the south west United States, Mali and India. They have designed and built loos at the Eden Project, a summerhouse for the National Trust, and had their work on show at the Chelsea Flower Show and in many exhibitions.
"All our work is about enjoying the material, and people seem to respond to it," says Jill Smallcombe, who trained as a sculptor and interior designer.
"We've worked with schools all the way through, even if it has just been going along for a day, or to make a bread oven or a seat," says Jackie Abey, who is an illustrator, painter and sculptor.
Now they have embarked on a more ambitious schools programme, helping schools build eco-buildings from cob. Called "The Three Little Pigs", it has involved building three classrooms with Devon primary schools – each one different – and a chicken house at the Farms for City Children charity, set up by the children's writer Michael Morpurgo. Schoolchildren also helped them to build a lakeside shelter, commissioned by the South West Lakes Trust.
Cob, they point out, is organic, local and readily available. And, when it has finished doing its job, it simply breaks down to earth again.
This fits squarely into the current agenda of schools, which have been told they must teach pupils about climate change and sustainability, and practice what they preach wherever possible.
"I was very into having an eco-classroom and a base for the children outside, and then Jill came along with the design," says Debbie Buckingham. The windows of her school's hut are made from recycled washing machine fronts, underlining the recycling message.
The artists work from an open studio at the back of Jackie Abey's farmhouse. Showing a visitor round, their passion for their subject exudes from every pore. They enthuse over the differing shades of cob blocks, explain how horse hair gives the material a different texture from straw, and recount where every heap of mud in the yard has come from.
One thing they aim to do with their art is to draw people's attention to all the many houses built from cob. There are still 40,000 cob buildings in the South West, as well as local variations in other parts of the country.
Their first big project was a coach stop and loos at an overflow car park at the Eden Project, where they had to work with 200 tons of sub-soil and train construction workers to handle this alien material. A lone man who stood watching them at the perimeter fence was later identified as the head of McAlpines. "We've educated an awful lot of builders about cob," says Jackie Abey. "Everything we've done has always had that educational aspect to it."
When they work with schools, pupils get different things out of it. One might spend hours perfecting a building's corner; another will happily push a wheelbarrow around all day. Quite a few are hesitant to get dirty. Modern children have freshly-laundered clothes and are unused to mud on their hands, but by the end of the day most are usually grime-streaked and grinning.
"We don't go there and teach them, we go there and play with them," says Jackie Abey. "The learning seems to happen by osmosis," adds Jill Smallcombe. "They watch things and ask us questions and just pick things up as we go along."
However, cob classrooms are not cheap. The Alphington project cost about £18,000, half of which came in grants and half of which it paid for itself. Three Little Pigs projects can last up to two years, with children helping to design the classroom and the project being woven into the curriculum before building even starts. "It's a big journey for the kids involved,' says Jill Smallcombe, "but it is also great for schools as it ticks so many different boxes."
Meanwhile, as artists, they safeguard their time in their studio and are currently making small cuneiform tablets – inspired by going backstage at the British Museum to handle 5,000-year-old examples of clay writing – and are also working with a collection of shelves and cabinets given to them by a local museum. The cob book library and cob-sealed drawers that are evolving as they respond to this haul of old furniture are as spooky and evocative as any installation to be found in Tate Modern, and highlight the infinite possibilities of their raw material.
"We have to be true to our passion, we have to keep that spark, or we wouldn't be sitting here talking about our work with schools," says Jackie Abey.
Much of their sculpture is inevitably transient. Outside the studio the figure of a kneeling woman is slowly returning to mud while work often has to be demolished after an exhibition is over.
Luckily, school classrooms are a different matter. Built properly with the "good boots and a good hat" that cob buildings always need, the artists say they should still be standing in 500 years' time.