Health reporter, BBC News
Around one in five teenagers are at high risk of depression
Teenagers are to be given group therapy in the classroom in an attempt to ward off bouts of depression.
A £1m government-funded trial taking place in Bath, Bristol, Nottingham and Swindon will provide sessions for around 7,000 adolescents.
The researchers hope the intervention will particularly benefit the 20% of teenagers at high risk of depression.
One expert said if proven, it would be a big step forward in dealing with depression in youngsters.
Everyone in the class will take part in the sessions which are designed to teach ways of coping with negative feelings.
But, ultimately, the idea is to prevent depression in those most likely to develop problems.
The programme is based largely on cognitive behaviour therapy (CBT) which aims to help people to pinpoint - and then change - thoughts and actions that cause emotional problems.
National guidance recommends CBT ahead of antidepressants in cases of mild to moderate depression and anxiety, and the government has set aside £173m to boost availability.
However, some experts have reservations about the effectiveness of CBT.
Pupils taking part in the study will be asked to fill in a short questionnaire to assess their mood and pick out any signs of depression before the sessions and again at six months and one year.
The trial, based on an Australian scheme called the Resourceful Adolescent Programme, which was designed to boost "mental resilience", will include nine sessions given as part of the pupils' Personal Health and Social Education lessons.
It will teach teenagers about keeping calm, problem solving, managing unpleasant emotions and looking at things from others' perspectives.
The results will be compared against classes receiving normal lessons and classes who will have "placebo" or dummy sessions.
Study leader, Professor Paul Stallard, an expert in child mental health at the University of Bath and an NHS consultant clinical psychologist said CBT had shown promising effects in children who are depressed or at risk of depression.
"We're looking at trying to give them useful skills as part of the school curriculum.
"And it seems to be a very useful way of getting to large numbers of children."
Professor Andre Tylee from the Institute of Psychiatry at King's College London, said up to 10% of teenagers and young people suffer from depression.
"NICE guidance highlights the need for early recognition and intervention as well as better prevention.
"This study will provide valuable information about whether group CBT can prevent depression in this age group."
The National Institute for Health Research, which is funding the research, is also planning other trials to "expand" the evidence on the effectiveness of CBT.
It includes investigating CBT for patients with depression who do not respond to treatment with antidepressants and a review of the clinical and cost-effectiveness of group CBT for postnatal depression.
Dr Tim Kendall, joint director of the National Collaborating Centre for Mental Health said the trial seemed to be well thought out.
"We don't have a huge amount of research to be absolutely sure about what is the right thing to do for kids who are depressed."
He added that only one antidepressant - fluoxetine - could be used in children and there were harmful effects associated with the drugs.
"If we find we can do this it would be a really significant step forward."