Many schools are struggling to find mental health services for pupils in need of support, headteachers have warned.
More than one in five school leaders who have tried to find professional aid for a youngster were unsuccessful, according to a poll.
At the same time, young people are bringing more worries into class than they did five years ago, it suggests.
The Government said it wanted to strengthen the links between schools and local NHS mental health services.
The survey, published by the National Association of Head Teachers (NAHT) and Place2Be, a children's mental health charity, questioned 1,115 school leaders across England and Wales.
The findings show that some 22% of those who had tried to commission mental health support for a pupil had been unsuccessful.
Of those who had tried to get help, more than half (around 57%) said that they found it difficult to find a service or professional to meet their needs.
And just under half (46%) said they found it difficult to work out what type of professional support they needed.
The most common barriers to getting help were a lack of capacity in services, no local services available and budget constraints, the survey found.
One school leader said: "There are not enough professionals to meet school needs across the country."
The vast majority of those polled (93%) said that pupils bring more worries into school than they did five years ago, while a similar proportion (96%) thought that children's abilities to learn are affected by the concerns they bring to the classroom.
Almost all (97%) of the primary headteachers quizzed said people underestimate the level of mental health problems among schoolchildren.
Catherine Roche, chief executive of Place2Be said: "In classrooms up and down the country, we know teachers are working incredibly hard to support the emotional needs of their pupils.
"They know when something is wrong, but it can be difficult to know how best to help, especially when there are no mental health professionals to turn to."
She added that evidence shows that making support available to children from an early age is beneficial to their wellbeing.
NAHT general secretary Russell Hobby, said: "Just as we are becoming more aware of children's mental health issues the resources are being taken away. School budgets are being cut by £3 billion so it will become increasingly difficult to fund in-school care for children unless these cuts are reversed immediately.
"This problem is exacerbated when the school seeks to access help itself, because of the chaos in the health and social care system."
A Government spokeswoman said: "As the Prime Minister announced last month, we want to strengthen the links between schools and local NHS mental health staff to ensure children and young people get the help and support they need and deserve. This is backed by a record £1.4 billion government investment to transform the mental health support available and school funding is also at its highest level on record."
She added that every secondary school is being offered mental health first aid training and there will be a major review of mental health services for children and young people.