Imagine teaching in a classroom where you don’t know until 9am how many pupils you will have, what age they will be or what language they will speak. Now imagine a situation where the sicker the pupil, the more you see of them.

This sad reversal of the normal scheme of things means that the children you get most attached to are often the ones most likely to die. This is a daily fact of life for the teachers of the Chelsea Children’s Hospital School.

In a small primary classroom, seven-year-old Sonny is sitting in bed reading while teacher Louise Egan stands beside him. Sonny is wearing only shorts and a large bandage. His leg, in traction, stretches out before him.

Sonny has been here five weeks and, despite his serious leg injury, has made great strides in his reading. He is wheeled away in his bed, an achievement sticker on his bare chest.

Under the leadership of headteacher Janette Steel, the hospital school has left behind the days of the “raffia mafia” - as hospital teachers used to be known - and made the education of sick children vital and forward-looking. Children here use new technology, have full access to the national curriculum and can study beyond their 16th birthday. They go horse riding, have links with European hospital schools as well as local primaries and enjoy therapeutic holidays.

Janette Steel must be one of few headteachers in London not to have her own office or even desk. She dashes from site to site through leafy Chelsea, in south-west London, conferring in strip-lit corridors with children, teachers, medics and the crucial fund-raisers (who make possible the post-16 provision). And she still finds time to teach.

Janette Steel is responsible for the overall vision of this school and its particular blend of compassion, professionalism and hectic eclecticism. A recent OFSTED report referred to her “exceptionally effective and inspirational leadership”.

“Inspirational” is an apt word, too, for the work of storyteller Roberto Lagnado. Mr Lagnado is like a character out of a folk tale, with his silver hair, bright eyes and comforting warmth. Three afternoons a week he tells the children myths, legends and folktales that look tragedy, death and loss full in the face.

“These people are dealing with death all the time,” he says. “I tell the story of Persephone, going into the underworld for four months of winter. It’s re-assuring. It doesn’t say you’re not going to die. But it says that death isn’t something taboo, or sterile.”

In exchange, children tell Roberto their own stories, of their lives and their families. “I meet some very, very angry people here,” he says. “Adolescents angry that they are going to die. Telling their stories gives them a chance to get rid of the anger.”

The important thing, he says, is not the stories he and the children tell each other but the intimacy the process engenders. “There’s a feeling of togetherness that builds up. Feeling that together you can fight,” he says. “People have been fighting all sorts of dragons, and some are conquered and some are not.”

The same bonds are forgedby the 20 staff at the Chelsea Children’s Hospital School, who work with varying numbers of pupils every day. As well as dealing with the full spectrum of age and ability, they also encounter almost every category of special need.

Each of the four school sites has a different atmosphere. The main site is the Chelsea and Westminster, a huge general hospital on the Fulham Road. Children may be admitted here for anything from orthopaedic surgery to sickle cell anaemia. Some are emotionally fragile as well as ill. Seven children this year have been reintegrated into mainstream schools after suicide attempts. Particularly with mental illness, education can be a part of the treatment.

Janine, 14, has been admitted twice this year. Although she was discharged from the adolescent ward a fortnight ago, she still attends the school. A pretty, tentative girl, she is using the computer for a portfolio she is putting together. She says: “If I get stressed out I’ve got a counsellor I can go and talk to. But I’ve got a goal to achieve now, and with the help of the teachers I’m getting there.”

Teacher Louise Egan, 23, thrives on the variety. “Pupils can range from a very able 11-year-old to a four-year-old. It’s a lot of preparation in the morning, but it keeps you involved. To get them back on their feet and wanting to go back into school is a good reward for me.”

School is not always the safest place to be, however. Some of her pupils have suffered dreadful injuries at the hands of other children; a seven-year-old girl was admitted last year with a broken arm and head injuries, sustained at school.

Nevertheless, children regard the hospital school as a safe haven, away from sometimes painful treatments. Sandra Grunewald’s daughter Maria, aged nine, has been in and out of the hospital all her life with a blood disorder. “Maria’s had 14 operations,” says her mother. “But she loves to come to the school. If she has an 11-hour operation, and after five days she says ‘Can I go to school?’, then I know she’s feeling better.”

Although the constantly changing pupil population requires flexibility, the education programme is meticulously planned. Teachers prepare a three-year curriculum map, plus schemes of work for a term or half-term, then adapt the material according to who arrives each day. Long-stay patients have their own weekly plans and are grouped with peers as much as possible.

For science teaching, the hospital itself provides a resource par excellence for both hospital children and others. Former deputy head Pat Aggarwal (now retired but still active in the school as a volunteer) devised programmes on topics ranging from Aids and contraception at key stage 3 to blood, bones and circulation for younger children, using the expertise to hand. Children come in from schools locally and from surrounding boroughs to do topic work.

Staff at Chelsea and Westminster make a point of inviting in schools attended by patients with long-term illnesses. Elaine, who has sickle cell anaemia and is frequently in hospital, was being bullied by her Year 6 peers. But when her class came in to the hospital recently she was the expert, talking on blood and showing the children round.

Medical procedures inevitably take priority over education in hospital schools, but the medical staff considers education important. “We want to get them back into the most normal routine possible,” says Jane Anderson, superintendent paediatric physiotherapist at the Chelsea and Westminster. “As soon as they’re stable, off they go into school.”

At Collingham Gardens, an in-patient psychiatric unit, other children are part of the solution, says senior teacher Amanda Ofield. “Before, they will have been the naughty child in the school, or the difficult person in the family. Here, everybody’s got something they’re working on. One of the best things is what children do for each other.”

The unit takes children between the ages of five and 13 for around four months for assessment and treatment. In the dining room, it’s stew or vegetarian rissoles and brown rice for lunch. Twelve-year-old Ben says he is studying the Tudors, Tibet and story-writing. All the children set weekly goals for themselves in consultation with teachers. Ben says that his is to “try hard and say three positive things about the day”.

Amanda Ofield says: “Children’s problems tend to be pooh-poohed by others. We’re in the business of acknowledging the seriousness and the reality of what’s going on with children.”

That approach certainly worked for one child - a girl who didn’t speak. Teachers began telling her in advance that she was going to be asked a question. Eventually, with the help of two other young patients, the little girl began to speak.

At the Royal Brompton Hospital - a specialist centre for heart and lung conditions - many children have long periods in hospital and recurrent admissions. Most are here because they suffer from cystic fibrosis, a life-threatening hereditary disease which tends to leave people chronically short of breath. While their peers feel immortal, these children inhabit a world of long-term illness and disability.

Teachers here - and at the fourth site, the Cheyne Centre, which treats children with cerebral palsy - do perhaps the most gruelling work. “You get very close to these young people,” says Janette Steel. “You see them achieve against the odds, you see their sense of humour and personality. The more they’re in, the more you get to know them - and the worse the prognosis.”

All the staff at the Chelsea Children’s Hospital School are affected by what happens to the children; they meet often to support each other, and use the services of two trained psychotherapists. The most distressing cases, says Janette Steel, are the children who undergo unsuccessful heart or lung transplants. “When someone has a transplant there’s a tremendous hope. When that goes wrong it’s worse because the hope has been snatched away.”

On a wall in the Royal Brompton is a huge, beautiful patchwork quilt, each square made by a different child. Janette Steel takes particular pride in this and other collaborative art works children have produced. “It’s a metaphor for being part of a bigger whole,” she says. “We’re not here to make them better. But what we can do is enrich the quality of their lives.”

Children’s names and some details have been changed