A small group of girls sits in a classroom in a post-GCSE crafts class.

Some, humming, paint flowerpots, and two others play a counting game called Zombinis on the computer. “School, sweet school,” reads a framed piece of embroidery on the wall. The air of normality is contradicted only by the girls’ heavily bandaged wrists and arms and a nurse sitting on a chair outside the door. The young women are all patients at the Priory, best known for its treatment of privately funded celebrity addicts, but also a major NHS provider of psychiatric treatment for young people.

The Priory Ticehurst House, outside the village of Wadhurst in East Sussex, has a long history as an asylum. Built in 1792, its wedding-cake exterior has become well known as successive waves of troubled stars have disappeared behind its doors for treatment, reportedly at a cost of up to pound;500 per night. The hospital has long been associated with the rich; from its inception, it was for the upper classes, who could bring their carriages and even take a villa in the 48-acre grounds for the duration of their retreat.

But the Priory group - which has an annual revenue of pound;120 million - is increasingly turning its attention to child and adolescent services. The group already operates four special schools in Cumbria and the West Country and has child and adolescent units in five of its hospitals. A new facility for 19 to 25-year-olds with Asperger’s syndrome and related conditions has just opened in Somerset and, from this autumn, Ticehurst House is to become an “adolescent campus”, given over entirely to the treatment of young people aged 12 and above and with a new specialist unit for children suffering from drug and alcohol addictions. “There is a growing need for adolescent services and they are going the way of specialisms,” says the services manager at Ticehurst, Ann Furminger.

The secluded location and sweeping grounds of the hospital are grand. But inside, Ticehurst’s wards and corridors feel less like the plush five-star hotel-plus-therapy of public imagination and more like a hard-working facility, with well-trodden carpets, smells of lunch and the occasional burst of loud teen music.

The hospital already has almost 30 patients. Two 10-bed acute psychiatric units cater for young people in mental health crises, usually at risk of killing themselves without emergency help. Here, staff treat depression, obsessive compulsive disorder, suicidal tendencies, and psychotic disorders. To be admitted, teenagers have to be seriously ill, with a complex range of needs. Many arrive in a desperate state. “It is rarely one thing and rarely straightforward,” says Ms Furminger. “They can present as very down, with no energy, no zest for life, isolated from their peers.

There may be problems at school or at home, and something may have happened in the past. Our job is trying to sort out that puzzle.”

Phoenix House, a unit in the grounds, has eight children - almost all of its patients are young women - who self-harm. “They don’t consider themselves to have a mental health problem,” says hospital director Belinda Malone. “They feel better about themselves having their own unit.”

These patients have usually been abused, are often caught in webs of chaotic relationships at home, and may also have eating disorders or be suicidal or involved in prostitution. “We give them tools to cope,” says Ms Malone. “We look at what leads up to an incident.”

Young people usually stay in the hospital for at least three months, and sometimes for up to a year; the aim is to turn them into “survivors, not victims”. Ticehurst House’s specialist addictions unit will cater for up to 10 young people. For the past few years treatment in the community has been the preferred option for young people who misuse substances, which means there are only 30 dedicated hospital beds across the UK. But, says Paul Thompson, manager of the new unit, “a large percentage of 11 to 18-year-olds come from such dysfunctional communities and families that a period away does far more than any community-based treatment could”.

The age at which young people are being admitted with addictions is getting lower, he says. “Twelve, 13 and 14-year-olds are getting addicted because of family problems. More young people parachute straight into the harder drugs - heroin and crack cocaine.”

The treatment begins, says Mr Thompson, with “unconditional positive regard for the young person. They quickly become authentic, and can then be authentic with themselves. I have never met one who did not want to succeed.”

One child in 10 in Britain has a clinically recognisable mental disorder, according to the Office for National Statistics. Twelve young people each week commit suicide, say the Samaritans, a rate three times higher than 20 years ago. Boys and the poor are most at risk. Children of families in unskilled occupations are three times more likely to have a mental disorder than those of professionals.

Yet it is girls who fill the Priory’s units. “Society is more accepting of young women being ill and coming forward for help,” says Mr Thompson. Boys, murmur staff, are more likely to end up in young offender institutions than hospitals.

Despite the suicide last summer of a GCSE student, staff at the Priory do not see a direct link between exam stress and young people’s mental illness. “Our numbers don’t shoot up at GCSE time,” says head of education Margaret Hardie. “But it may be the last straw - that extra bit of stress.”

Consultant psychiatrist Dr Hartmut Steffen says vulnerable adolescents are always more at risk of being knocked off their life course than older people. “If we are lucky,” he says, “youngsters develop a vision of their own life. That is a joyous day.”

The school is an important part of the treatment on offer at the Priory; part of its function, says Ms Hardie, is to give the children some ordinary time. “As teachers, we have to be aware that this is a hospital and that these young people are ill. At the same time, we try to keep school as normal as possible for them, because it can be the only normality.”

School takes place every day from 9am until 12.30pm and discussion of illnesses is discouraged. Providing education for the Priory’s young patients is challenging. Many have not attended school for years and some are school-phobic. They have all levels, “from some who find it difficult to write a sentence right up to others who have top A-level grades”, says Ms Hardie. She trained in Scotland and worked in a mainstream school there before joining the Priory six years ago.

Flexibility and a sense of humour are the best qualifications for the hospital’s teachers, she says. “Things change quickly. And humour builds up a rapport.”

English teacher Jennifer Pearce, who joined the staff from a nearby boys’

school, says: “Here, you feel much more effective and can address problems.”

The seven part-time teachers and two classroom assistants cannot run a full curriculum. The focus at GCSE is on maths, English and science, plus music and art. There are few sports facilities, but there is plenty of space in the grounds for walking, cycling and letting off steam. Around two-thirds of the patients are Years 10 and 11. The centre offers A-level English, maths and biology, but in other subjects can only help students with work provided by their schools. Alongside the main school is a “skills centre” for the over-16s and for school-phobics who need to reacclimatise to life in the classroom.

Many young people are working towards re-integration in their own schools.

Staff encourage trips to nearby Tunbridge Wells, walks to the village and even work experience at local businesses for those nearing the end of their stay. Most go home at weekends; working with families is part of the programme. Priory school staff contact patients’ schools (“in general, they are very helpful”, says Ms Hardie) and make individual plans for them.

Charge nurse Mairwen Mullally says that when the young people do go home, they sometimes find that being at the Priory “can have a weird sort of street cred. Many find that their peers have had distressing experiences too, and can empathise.”

A new, larger school will open in Ticehurst House this winter, with bigger classrooms, more teachers and better resources. Managers are aiming for DfES registration, to enable the school to take day students with special needs.

But while mornings are for education, afternoons are for therapy. Children must work hard at the latter, with their own individual therapists and in groups focusing on, for instance, emotional regulation, social skills and self-care. “They can observe their own thinking, emotions and behaviour,” says therapy manager Sally Sedgwick.

“They learn how to calm themselves down, step back, be more in control.

They keep diaries of their emotions, urges, whether they have self-harmed.

Then they analyse what skills they tried to use or could have used. If a young person is having problems or takes a dislike to someone, we say, ‘That’s great. Let’s learn how to manage this’.”

Seventeen-year-old Susie’s shared bedroom is a testament to the work she is doing on herself. While her roommate’s wall is plastered with pictures of horses, Susie’s is covered in diagrams showing “reasonable mind, wise mind, emotional mind” and a distress management plan to remind her of distraction techniques for when depression hits - such as, “play pool with all your five senses and focus”.

Susie left school without GCSEs after being severely bullied, but is taking English and science in January. “I have been to many hospitals and this one has the best school,” she says.