School is the biggest source of stress for young people, according to a new survey by children's charity Barnardo's.
The charity found that almost half of children aged 12 feel sad or anxious at least once a week, with the proportion rising as they get older.
Barnardo's commissioned YouGov to examine what is troubling young people and how they might be better supported.
According to the survey, nearly half of 12-year-olds in England (48 per cent) feel sad or anxious at least once a week. By the age of 16, 70 per cent report feeling this way at least once a week, with more than a fifth (22 per cent) having negative feelings as often as once a day.
Twelve-to-16-year-olds report that school is their main cause of stress (65 per cent), followed by thoughts about their future (42 per cent), problems at home (31 per cent), their weight (26 per cent) and offline bullying (25 per cent).
By the age of 16, stress at school is a worry for 83 per cent of children in England and 80 per cent are worrying about their future.
The survey found that social media has an important effect on mental health for young people, with 11 per cent reporting that they worry about getting enough "likes" or responses to posts.
Twelve per cent say they are concerned about online bullying, while 15 per cent say they have been troubled by something they have seen on social media.
According to the survey, which was commissioned to mark the beginning of Children’s Mental Health Week today, three-quarters (75 per cent) of 12-to-16-year-olds think it would be helpful if they had a counsellor or another professional at their school to talk to when they were feeling down.
When asked who they would talk to if they felt sad or anxious, 38 per cent said teachers, 63 per cent said friends and 71 per cent said family members.
Barnardo's chief executive Javed Khan said: "It is deeply concerning that so many children in England are growing up feeling sad and anxious, and these feelings are intensified as they get older.
"Although these can be normal emotions experienced while growing up, children need support to deal with the pressures of everyday life.
"We need to create a culture where everyone has a greater understanding of what keeps children mentally well and when professional help is needed."
He added: "We want parents and carers to be confident in recognising if their children are unhappy, and teachers and other professionals to be sufficiently trained, adequately resourced and available to support them."