It began three years ago with a swollen knee. At the time, Clare Allen and her parents put it down to a netball injury. But, within a matter of weeks, the active teenager who loved nothing better than representing her school in sports tournaments and dancing competitions was having difficulty getting out of bed. At the age of 13, Clare was quite literally seizing up.

The stiffness that had made her left knee painful to bend soon spread to her right knee. And before long her hips and elbows and even her fingers were inflamed and swollen. All the evidence pointed to rheumatoid arthritis, a devastating diagnosis that was confirmed by blood tests.

But could this be possible? Could a healthy, sporty schoolgirl just starting to study for her GCSEs really be disabled by what most people still think of as a disease of old age? In fact, one in every 1,000 children in the UK has juvenile idiopathic arthritis (JIA), an umbrella term now used to describe any of the many forms of arthritis which can afflict people from an early age. For these children, previously normal tasks such as walking, writing or riding a bicycle suddenly become difficult and painful or even impossible.

They and their families must also adapt rapidly to the grim realities of long-term illness. Powerful drugs, many of which have unwelcome side-effects, become part of the routine, as do exercise regimes, hospital visits and even surgery. Clare’s doctors prescribed non-steroidal anti-inflammatory drugs, and methotrexate, a drug originally developed to tackle leukaemia. She had to get used to giving herself injections once a week in order to avoid the sickness that resulted from taking the medication in pill form. And because the drugs impair her immune system, she has had to cope with a succession of infections, ranging from pleurisy to glandular fever. All of this pain and disruption is on top of the normal struggles of teenage life - namely, growing up, going to school and studying for exams.

Inevitably, Clare, who lives in Harlow, Essex, was teased by her classmates for having what they, in common with most adults, regarded as “an old lady’s disease”. Next thing, they said, she would be turning up with a walking stick. “They were not trying to be horrible or bullying or anything,” she says, “but the jokes did hurt my feelings. Then some would be fascinated by it, and when I met new people, they would say: ‘This is Clare. She’s got arthritis.’ And I was thinking: ‘Thanks for mentioning that first, before they even get to know me’.”

Soon, other problems began to surface, such as the attitude of Clare’s school to her disability. “To begin with,” says her mother, Kim Allen, “they were helpful and understanding. Clare was allowed 25 per cent extra time when sitting exams, and her special needs co-ordinator was really supportive. She was even given an early dinner pass so that she could have her pick from the menu.” But, on a couple of seemingly petty issues, Clare’s parents quickly found that the system was even less flexible than their daughter’s limbs. “It sounds like a silly thing, but we had a big battle to get duplication of books,” says Ms Allen. “Because some of them were so heavy and she had trouble carrying them, we wanted Clare to have separate sets at home and at school, and a locker to keep them in. But it took a long time and a lot of nagging.” Clare sometimes had to stumble around the school in search of a spare textbook because a teacher had forgotten to bring her copy along to the classroom.

If heavy textbooks were a minor irritation, a row about trousers was to prove critical. Skirts were compulsory for girls. But when Clare was first diagnosed, she had problems putting on tights and doing up buckles. On cold days, her knees were particularly painful. And besides, she felt self-conscious wearing a skirt when her joints were visibly swollen. A doctor’s note requesting that she be allowed to wear trousers on cold days seemed to fix the problem.

But one year after being diagnosed, she was summoned to the deputy head’s office and told that she would no longer be allowed to wear trousers but would have to make do with thick woolly tights and a long skirt. “They had been approached by four Muslim families who had demanded that their daughters be allowed to wear headscarves and trousers,” says Ms Allen. “But the governors had decided that this wasn’t appropriate and that all the girls should wear skirts, regardless of the fact that, in Clare’s case, it was a medical dispensation rather than religious conviction.

“It all got very silly, and we had a long chat with them. Clare’s paediatric occupational health therapist also spoke to them. But they weren’t prepared to give way at all. Her consultant couldn’t believe it, but as parents, we didn’t want to push it too far for fear that they would then get petty-minded in the classroom.” Which is why, with an impressive string of GCSEs under her belt, Clare has now moved to another school to prepare for her A-levels. But her mother remains bitter about what she regards as a lack of sensitivity. “The whole ethos of the school was wrong,” she says. “If they couldn’t make the effort to care about Clare, then they really don’t deserve her.”

Neil Betteridge, acting chief executive of the charity Arthritis Care, and himself a former sufferer of JIA, believes that other people’s ignorance can be as disabling for sufferers as the disease itself, particularly for children. “Imagine if that school had banned a wheelchair,” he says of Clare’s experience. “In my case, the arthritis only disabled me for the first few months when I couldn’t get out of bed. After that, it wasn’t so much the illness as the layout of the school and the way we organise our society that impaired me.”

Given that one in five of us has arthritis, making it the biggest cause of physical disability, Mr Betteridge believes that schools should be educating all children about the disease, “as they will either get it or come across someone who has it at some time in their life”. But it gets no mention in the curriculum, and children with arthritis find themselves “constantly having to educate people”. Which is why Arthritis Care has joined forces with the Children’s Chronic Arthritis Association and the Lady Hoare Trust for Physically Disabled Children to form the Chat (Children Have Arthritis Too) Alliance, which has produced a series of information booklets for teachers and parents.

After three years with arthritis, Clare has learned to tell as soon as she wakes up in the morning whether she is facing a good day or what she calls “a stiff day”. And she is painfully aware that many of the people she runs into at school will still be confused by the fact that she can appear relatively agile one day and virtually immobilised the next.

But at 16, and with A-levels on the horizon, she now has an important decision to make: whether to risk giving up methotrexate. In some cases, the drug can break the cycle of inflammation and even force the disease into permanent remission. And for some time now, Clare’s joints have been so improved that she has even been able to resume her basketball and dancing. “The trouble is,” she says, “that if I come off it and then have a major flare-up, the drug would not be so effective the second time round.

So coming off it is like a leap of faith. There is no way of knowing what will happen. But if it does come back, I’ll deal with it. I’ve dealt with it once, so I’ll deal with it again.”

The theme of this year’s Arthritis Research Week, which ends today, is back pain. For more information: the Arthritis Research Campaign (Arc): 0870 850 5000; www.arc.org.ukArthritis Care: 020 7380 6500; www.arthritiscare.org.uk. Includes details of complementary therapiesChildren’s Chronic Arthritis Association (CCAA): 01905 745595; www.ccaa.org.ukThe Lady Hoare Trust can be reached via Contact a Family on: 020 7608 8700; www.cafamily.org.uk