School nurses could help tackle the "crisis" in children's mental health but numbers are falling, a union has warned.
At the start of its annual conference in Glasgow, the Royal College of Nursing (RCN) said more investment was rapidly needed to shore up school nursing.
Its poll of school nurses found many felt their workload was too heavy and 28% work more than their contracted hours every single day.
The number of school nurses in England has fallen by 10% since 2010. There are now 2,700 who care for nine million pupils, according to figures from the Health and Social Care Information Centre (HSCIC).
Data from the charity Young Minds shows that one in 10 children and young people aged five to 16 suffer from a mental health disorder - around three children in every class.
Between one in 12 and one in 15 children and young people deliberately self-harm and nearly 80,000 children suffer from severe depression, of which more than 8,000 are aged under 10.
The RCN said school nurses are perfectly placed to pick up on issues and help children suffering mental health problems.
Of 277 school nurses surveyed by the RCN, 68% believed there were insufficient school nursing services in their area to provide the care and support children and young people need.
Seven out of 10 said their current workload was too heavy, while almost a third (30%) said admin took up most of their time.
Almost four out of 10 (39%) said they had too few resources to do their jobs effectively.
Janet Davies, chief executive and general secretary of the RCN, said: "Mental health is a mounting problem for children and young people.
"There are huge variations in care across the country and far too many vulnerable children are just not getting the support they need.
"School nurses have the skills and the experience to provide a wide range of mental health support, from counselling to promoting healthy lifestyles. But, as our survey shows, there are too few of them, and they are simply too stretched.
"All children deserve access to the right care, in the right place, at the right time.
"Only by investing in school nursing and wider mental health services, can this crisis be tackled and children be given the best chance possible of leading happy and healthy lives."
A Government spokesman said: "Children's mental health is a priority for this Government and we are putting a record £1.4 billion into transforming the support available to young people in every area of the country.
"This funding will help recruit more staff and create improved training that school nurses can access.
"We are working with NHS England to strengthen the links between schools and mental health services through a £3 million pilot, and are investing £1.5 million on developing peer-support networks in schools."