Just over one in 10 children in British secondary education say they have no one to talk to if they feel worried or sad while at school, a survey has found.
Thirty-eight per cent of the 1,323 schoolchildren between 10 and 15 polled for the research said feeling worried or sad made it difficult for them to fall asleep.
More than a quarter (26 per cent) said it also meant they struggled to keep up with their homework, according to the study commissioned by the Mental Health Foundation (MHF).
Twenty-seven per cent of youngsters said they tended to get into fights or arguments when feeling worried and sad, and the same number said they tended to prefer to be alone.
"Our survey provides shocking further evidence of the growing crisis in the mental health of children," said Antonis Kousoulis, associate director at MHF.
"Nearly half a million children in the country have no one to speak to at school when they are experiencing feelings of sadness or worry. That is plainly unacceptable.
"We know there are many schools that are doing excellent things in this area, often in difficult circumstances, but this needs to keep improving... If we are not tackling mental health problems early, then we risk failing the next generation right at the start of their lives."
The survey comes amid a flood of reports warning the UK's schools are failing to keep up with ballooning demand for children's mental health services.
The National Audit Office warned on Tuesday that government estimates of demand for pupil mental health services may have dramatically underestimated the scale of the problem.
A survey by the Education Policy Institute released this week also found that referrals to child and adolescent mental health services have jumped by more than a quarter in the past five years, and tens of thousands of children are falling through the gaps.
Mary Bousted, joint general secretary of the NEU teaching union, said in a blog post that pupils were facing an "unrelenting treadmill of academic pressure".
"A perfect storm of cuts to local authority support services, cuts to school funding, increased exam pressures and a narrowing curriculum offer has created a crisis situation," she added.
MHF based its findings on Office for National Statistics data, which estimates there are 4,292,784 10 to 15-year-olds in Britain, 97 per cent of which are in school.
If 11 per cent say they have no one to talk to, that would equate to 476,066 children.