In a sunny library, Karen Draysey introduces one of her pupils, 14-year-old Belma, who is looking forward to our chat. What would we like to talk about: line one or line two?

Belma’s eyes shoot to the right as she chooses line one with the only muscles she can use to communicate. Ms Draysey flicks to the activities page in a fat book of symbols she’s holding for Belma. Line one, line two? Again the eyes dart. Right for no, left for yes.

And so on, until we are talking about music videos, specifically Patrick Swayze in the film Dirty Dancing. It has taken quite a long time because I keep guessing wrongly and causing Belma to frown slightly. She is used to much more skilled interlocutors.

But finally, yes, I know the film and Patrick Swayze is such an exciting dancer. The eyes smile. She becomes so enthusiastic that her mouth seems to lock open and her teacher gently closes it. Belma’s eyes are suddenly damp, but then they smile back at Ms Draysey.

“I enjoy getting to know a child,” says Ms Draysey, who has been teaching here at Chailey Heritage, an independent special school outside Lewes, in East Sussex, for nine years. “You don’t get that in a class of 20 or 30. We know the home background; we know the whole child all the way through. Some of them are here from nursery to 19. You really have to tune into them.”

Ms Draysey’s class contains six children aged 13 to 14, all but one seated in customised wheelchairs around a central table. John, who has muscular dystrophy and came to Chailey from a state primary at the age of nine, is the only one who can talk. He shows me his humanities folder, and his work on the Second World War - some writing and drawings, maps, and diagrams. He talks like any other 13-year-old about what he likes and doesn’t like about school. “We made this recipe,” he says, pointing to a sheet about wartime cookery. “It’s all carrot, but it tastes like apricots.”

His mood towards his classmates today is one of exasperated tolerance. He could hardly be in a setting more different from his primary school, but he is heavy and immobile and his condition is progressive. Belma, airlifted out of Bosnia as a baby, was disabled by meningitis, and the other four children all have severe cerebral palsy. Timmy - the only walker in the class - smiles a lot. He spends one day a week at a nearby secondary school, Ringmer community college, and he uses his own sign language to explain that he bakes cakes while he’s there. Timmy can hear, but he cannot speak.

A handful of the 100 pupils at Chailey are relatively cognitively able, but the range is enormous and so is the challenge to those who teach them, yet turnover among the staff of 23 teachers and 47 classroom assistants is low.

There are 200 staff in all, many part-time. The children - roughly a third day pupils, a third day pupils with some respite care and a third weekly or fortnightly boarders - come from local authorities in London and the Home Counties. Fees average around pound;32,000 a year.

“You’re rarely doing something that everyone in the group can do, so differentiation is important,” says Karen Draysey. “Take a lesson on the Second World War. Although we work mainly to P-levels (pre-national curriculum levels), we are delivering a modified version of the national curriculum. Charley and Chris would listen to some songs by Vera Lynn and Glenn Miller, and we would ask whether they were happy or sad songs. At the other end of the scale, I would expect more from John.

“It’s a multi-faceted job. You’re dealing with switches and equipment, with children who need to be made physically comfortable, as well as with straight education. It’s a multi-sensory and creative approach to teaching.

We do lots of modelling, making and painting.” It’s also very hands-on for staff, with a physical dimension absent in mainstream education. Teachers routinely wipe wet chins and move stiffly outstretched arms that are in danger of banging against door frames as wheelchairs pass through.

The children are frequently taken out of class for medical treatment, to use the toilet or to have equipment fitted. Uniquely, Chailey shares its site with an NHS special unit called Chailey Clinical Services, which treats pupils and outpatients, mainly those with brain damage. Dr Elizabeth Green, clinical director of paediatric rehabilitation, believes there will always be severely disabled children to care for - including those born very prematurely or the victims of road accidents - and that having a school on site is “a wonderful thing. There’s nowhere in the world like it.” Now all children are deemed to be educable and Chailey sees itself as a centre of expertise and research - in health and education.

The emphasis is on helping each child find a voice, whether through a computer, operated by the child’s strongest movements; a sign language, such as the 1,600-sign Signalong; or by using the Chailey Communications System book. An electric trackway in the school grounds lets children control their own wheelchairs by touching switches on a keypad. And Chailey technical wizards are working on a voicebox that will be talk to the child, perhaps in his or her mother’s voice, saying, for instance: “You’re at the hall now, Jimmy. Turn right to go in.” Auditory signposting is especially useful for children with poor vision.

“You need loads of patience,” says teacher Patricia Bruce. “The patience to work out what the child is feeling. No one can learn unless they’re comfortable. And these children are held in their chairs at several body points. Progress is slow and small. You have to watch for it.”

Outside, the school’s music staff are playing guitars for a group of pupils who dance in wheelchairs pushed by helpers. There is a lot of music and laughter at Chailey. When the day pupils arrive in minibuses each morning, in beautiful surroundings on the edge of Ashdown forest, they are greeted by adults who joke with them - “I like the spiky hair, Josh”.

Chailey Heritage celebrates its centenary this year. The school dates back to a time when disabled children, many with TB and rickets, were in rural, sometimes isolated, institutions. Chailey has plenty of space and the buildings are a mixture of 1930s arts and crafts, ultra-modern, and 19th-century tile-hung traditional Sussex.

For today’s teachers and pupils, all that is a mixed blessing. The loveliest of the original buildings require well over pound;1 million to renovate - a huge task facing the school governors led by Verena Hanbury, granddaughter of the original founder, Dame Grace Kimmins. As for the tranquillity - well, Belma’s eyes navigate Ms Draysey through her copy of the Chailey Communication System to tell me she likes clothes and nice colours. During her residential weekends, she would choose to be pushed to some shops if only there were any nearby.

Today, this sort of school, with its 100 pupils and 200 staff, might well be in the heart of a community. Then again, children born with Timmy’s less severe level of disability might be educated in the mainstream from the outset.

The evolution in special education, which has brought Belma and the rest of the pupils to this quiet spot, goes on, says Chailey’s headteacher, Alistair Bruce. “Inclusion is the biggest single issue in special education today,” he says. “Nobody owes us a living, after all; and the mainstream is now much more able to include disabled children. Local education authorities understandably want to use their own provision rather than refer to us.

“For many children, though, this is the best place. We also have some whose parents feel they are better off, perhaps safer, better understood, here.

And, of course, we have the clinical services and a rehab engineering workshop for on-site orthotics.

“We could, some time in the future, move towards the autistic spectrum of children, which is hard for mainstream to accommodate, but you couldn’t introduce those children now alongside our present pupils. We need to try to provide what local education authorities can’t or won’t, complementing their work rather than being in conflict, which helps no one.”

Play is as important as the curriculum. As well as stables and a swimming pool, there’s an adventure corridor where children can use a pad on their wheelchairs to make a bear blow bubbles, create puffs of air or make fountains. Technology has revolutionised education and play for this group of children.

“A key part of our on-job training is to make sure staff see a child, not a wheelchair. You may be teaching at a cognitive level of seven or eight with a child who is 16, and who has a 16-year-old’s interests. For a party, they want to dress up for a prom, not have games, jelly and ice cream,” says Mr Bruce.

“Child progress is a teacher’s fuel and, yes, sometimes you go home from this job with your tank empty. But it’s about knowing the child so well that you know all the nuances and you can tell a child a joke, look in his eyes and know he’s got it. That’s your fuel.”

Chailey Heritage - a hundred years, by David Arscott, is published by SB Publications at pound;12.50