One in 10 teenage girls harms herself each year, according to the largest study of self-harm among teenagers in England.
The survey of 6,000 15 and 16-year-old pupils in Oxfordshire, Northamptonshire and Birmingham reveals that the problem is far more widespread than previously thought.
Girls are four times more likely than boys to harm themselves, with 11 per cent of girls and 3 per cent of boys reporting they had done so in the previous year.
The findings, from a study carried out in 2000 and 2001, have been published in By Their Own Hand, a book providing practical advise to help adults to detect and advise young people at risk of cutting or otherwise hurting themselves.
The researchers, led by Keith Hawton of Oxford university, believe the vast majority of self-harming incidents go unreported as just 13 per cent result in hospital visits.
The Mental Health Foundation has also published a guide for teachers.
Celia Richardson, its author, said teachers and other adults working with children often failed to realise that self-harming was a coping mechanism for dealing with stress, such as bullying or problems at home.
When Sam Pearson began to slash her arms, thighs and chest at the age of 12, teachers said she was a bad influence on other pupils.
The PE teacher at her Newcastle comprehensive spotted the scars. "I got dragged into her office," Ms Pearson, now 21, said. "She said, if you ever need to talk, it won't go any further. Then she told my parents."
She said teachers should deal with it tactfully. "My English teacher was the only one who treated me like a human being. She talked to me, rather than at me.
"If schools don't handle it properly, it just gets worse and worse."
The Truth About Self-Harm is available free from www.selfharmuk.org or www.mentalhealth.org.uk. By Their Own Hand is published by Jessica Kingsley, priced pound;17.99
How to spot and help someone who is self-harming
* They may be withdrawn, depressed or secretive, and spend a lot of time alone
* Lack of energy
* Wearing inappropriate clothes, such as long sleeves in hot weather
* Avoiding activities where they need to expose flesh, such as PE or swimming
What to do
* Focus on the underlying problems
* Recommend counselling
* Suggest distraction methods such as: using a red felt-tip to mark where they might cut;
hitting a punch-bag;
rubbing an ice-cube across the skin;
putting elastic bands on wrists and flicking them; exercise;
writing down negative feelings and then ripping up the paper