Jessica Hargreaves isn’t allowed to talk to her friends anymore. She suffers from cystic fibrosis, a genetic condition that affects the lungs and digestive system, and she is even forbidden from mixing with other cystic fibrosis patients at the Royal Brompton Hospital, in case she catches an infection.

But thanks to the pioneering approach of the Chelsea Children’s Hospital School, which teaches dozens of children across four sites including the Royal Brompton in London - burns victims, those with cystic fibrosis, heart conditions and psychiatric patients - the 13-year-old is able to join in classes with her friends via video-conferencing.

The system allows children to listen and participate from their hospital bed, and even take the form of 3D avatars such as a dinosaur, shark or cat. “I like English. I’d like to be a writer or an actress,” Jessica says.

The Chelsea School is one of 26 in the country that teach sick children. It employs about 40 people, of which more than three-quarters are teachers, and offers an energetic and demanding environment for staff. “You need a good pair of trainers and a sense of humour,” says Bella Sood, 35, head of science and maths. “You have to be flexible and versatile. I also teach technology, RE and history. We talk to the patients’ normal schools so that when they come in they start where they left off.”

Pupils stay at the hospital for an average of two weeks, but sometimes for more than a month. In those cases, or where they have been admitted in an emergency, they may find themselves revising for or even taking exams in hospital.

Paula Cogan, 14, has received a string of Level 6s in her key stage 3 tests, doing much of the revision at the Brompton. “I did quite a few practice tests. When you come in at first it’s a bit unsettled, but everyone’s really nice and I’ve made lots of friends,” she says.

Sats aside, teaching at a hospital school is a different experience, with much smaller classes and time to focus on the individual child. Caroline Pearson, 27, who taught for three years in mainstream education before joining the school, says: “I used to like my routine, but every day is different here. We have to wipe everything down and we’re not allowed to use wooden toys or puppets because they can carry germs.”

Illness rarely intrudes on classroom life, she says, apart from the regular interruptions for X-rays and treatments. “We don’t mention illness in the classroom unless they bring it up. It’s a chance for them to get away from that world. We have had children pass away and it’s sad, but pupils rarely ask about it.”

Over at Collingham Gardens, a 10-minute drive away, staff are grappling with a different set of issues. Here, they teach children from the ages of seven to 14 with a wide range of psychiatric conditions including anorexia, bulimia, Asperger’s, post-traumatic stress and Attention Deficit Hyperactivity Disorder (ADHD). But Miranda Duurloo, 42, a teacher, has little time for medical labels. “I don’t read the file unless I need to. I prefer to let the children introduce themselves to me.”

Stories of the improvements children have made while passing through the unit are extraordinary. One child, an 11-year-old with a diagnosis of ADHD, came in displaying aggressive behaviour, verbally and physically.

“He had never experienced any success at school so he had to sabotage the lesson before he was asked to do anything,” says Miranda. She noticed he had a passion for cooking (his ambition was to be like Jamie Oliver, the TV chef) so she incorporated this into every lesson. “His reading would be recipe books and his prizes would be cooking utensils. Then we took him to Fifteen (the Jamie Oliver-owned London restaurant designed to give underprivileged young people training in the catering industry) and he was so excited. For the first time he could see there was a future for him.”

Another child, a 12-year-old boy who had emigrated from Africa, was referred to the unit by his school as a suicide risk. They discovered his family, who had suffered severe trauma as a result of seeing relatives murdered in a civil war, believed he was possessed and made him dress and prepare food alone. Now, he is in the care of a foster family and recently returned to the unit while making a project about the history of his life. “Sometimes when you find out about these children’s lives you think, ‘thank God they’ve got these coping strategies.’ At least they’re fighting back,” says Janette Steel, the headteacher.

Another boy, who was diagnosed with Asperger’s, physically attacked two teachers and was rejected by 23 schools, but is now studying theology at Oxford.

“He came back to visit and said, ‘I never had Asperger’s. I was only pretending.’ I said, ‘You’re a good actor’,” says Janette.

The unit, situated in a rambling red-brick Victorian building near Gloucester Road in south-west London, is papered in displays, photographs, artwork and scattered with pottery. It’s ethos is about boosting children’s self-esteem by emphasising their achievements, constantly praising and rewarding them if they try - although they may not always succeed - to behave well.

In an English lesson upstairs, children are discussing the impending visit of Charlie Cox, a local actor who featured in Stardust, the fantasy film starring Robert De Niro. “Has he got muscles?” one girl wants to know. Another boy, diagnosed with Asperger’s, is more preoccupied about when he can return to a normal school. “If I’m not being sick, why am I in a hospital?” he asks.

The teachers agree that rigorous planning and the ability to think on your feet are essential. “You’ve got to be authentic and consistent,” says Melissa Hind, 24, who teaches at Collingham. “The children are very perceptive and they quickly see through anyone who’s a fake.”

What’s it like teaching in a hospital school?

“I did six years in mainstream. I was head of science and maths in a Birmingham school but I wanted to do something different. I was surfing the internet and just by fluke happened to see an article about hospital schools. I speak four languages, so the multicultural element of working in a London hospital appealed. It seemed to suit me down to the ground. Fourteen people came for interview (competition for posts in hospital schools tends to be quite intense) and I was lucky enough to get the job. I still teach two days a week at a local college and it keeps me up-to-date with changes in the curriculum. Local authorities are obliged to give children a continuous education if they’re off for more than 10 days. We tend to pick up where children left off but also have our own schemes of work if this isn’t possible. We are a registered exam centre and have had children taking exams after being sick or at an hour’s notice after being rushed in with a broken leg.”

Bella Sood, 35, head of science and maths

How to get a job in a hospital school

- Hospital schools advertise vacancies on their websites and in The TES.

- Staff numbers and class sizes are smaller and there can be more scope for part-time working.

- Activities can continue into the evening so be prepared to be flexible.

- Competition for places is often strong. Good subject knowledge and adaptability are musts.