One in every 15 young people in Britain self-harm, a recent report concluded. Statistically, that means each classroom in Oxfordshire contains one or two self-harmers, usually young women who deliberately cut into their skin with a razor blade or other sharp implement to make themselves feel better. And it is on the increase. Children's author and English teacher Joanna Kenrick is trying to change this through her latest book, Red Tears.

Teenager Emily Bowyer is 16; pretty, hard-working and from a stable, middle-class background. She starts self-harming because she cannot cope with exam pressure, the usual teenage conflicts with her mother and the way her friends ostracise her. As self-harming becomes addictive, her mental health deteriorates, her arms are covered in scars and eventually her behaviour is exposed. The rest of the book then deals with the fall-out.

On the surface, Emily seems to have nothing wrong with her, which is partly why Joanna created her. "That's the most difficult type of self-harmer to understand," she explained. "Many self-harmers have gone through very traumatic events, but a lot of them haven't." Communicating with a group of self-harmers via the Internet, Joanna said many who wrote to her expressed guilt, because they didn't seem to have any reason for their behaviour.

"It must be awful to feel like that, to feel that your problems are not worthy of someone else's sympathy," Joanna said. "No matter how small the problem is, if it's causing you stress, then it's something you need help with sorting out."

She has never been a self-harmer. Before she came cross the topic at the age of 26, she had no idea that the condition existed. Now 31, she researched the book by joining self-harm web forums. I suggested that the Internet must help sufferers feel less isolated. "It's a double-edged sword," she said.

"You realise you're not alone, which can be hugely reassuring and you don't feel like a freak any more. Other people are like that and they can't all be bad.

"But if you spend a lot of time talking to other people who self-harm, then you begin to regard it as acceptable and normal, which is dangerous, because then you stop looking for a way out."

The book includes a warning to self-harmers, that they may find parts of the book trigger their condition, and must take responsibility for their own safety. She said: "The editor at Faber and I felt very strongly that since, hopefully, a lot of self-harmers will read this, it was irresponsible of us not to warn them about the descriptions in the book. If you are a compulsive self-harmer, it's a bit like being addicted to drugs. If people start talking about drugs, then you want it." She even wondered whether she should write the book at all, and asked the opinion of self-harmers, whose opinion was split 50-50. Some said there should be more books about this kind of thing, so that people understood, while others thought it put ideas into the minds of vulnerable people. She and her editor are conscious of their heavy responsibility. "I think we all feel that we need to get it right and don't want to ripple the waters anymore than is necessary."

Joanna describes herself as a problem-solver. After an Internet conversation with an American girl, where she ended up feeling rather exasperated, she realised she could never be a counsellor.

"If you bring me a problem that you don't want me to solve, what do I do?"

Yet through articulating Emily's experiences, Joanna is certainly solving one problem - how to get outsiders to understand why young people self-harm. She also wants to show self-harmers that there is a way out and that they are not alone.

Joanna was brought up in Appleton in Oxfordshire, and left to train as an actress in London and then as a teacher. After three years in a North London comprehensive, she returned to Oxfordshire three years ago and lives in Long Hanborough, teaching English part-time at Wychwood School in Oxford.

She also writes books for reluctant readers, concentrating on teenagers with a reading age of under eight. "Not many people can write that, as it's very difficult to find gripping stories in 2,500 words - short sentences, nothing complex, no complex vocabulary," she said. "I actually find it quite liberating. I like writing to limits."

Although Joanna would like to write full-time, she enjoys teaching English. "I enjoy passing on an enjoyment of words," she said. "Even if you're not sure what the words mean, then you can enjoy the sounds." She also likes turning pupils on to reading in general.

"I make it my mission to find books that everybody loves," she said. "Because there are so many books out there and not enough time to read them all."

Red Tears is published by Faber at £6.99 as part of National Self-Injury Awareness Day.