Children as young as four suffer from panic attacks, anxiety and depression while at school, a new poll has shown.
Nearly a fifth of teachers asked by the NASUWT union said they had been in contact with four to seven-year-olds showing mental health issues, and 35% had seen problems in youngsters aged seven to 11.
Almost all teachers (98%) said they have come into contact with pupils who are experiencing mental health issues.
NASUWT general secretary Chris Keates warned there is concern among teachers about a gap in the availability of experts and counselling to help children with mental health needs.
The Department for Education (DfE) said it was "strengthening links" between schools and NHS mental health staff.
Nine in 10 of the 2,051 NASUWT members polled said they had experienced a pupil of any age suffering from anxiety and panic attacks, while 79% were aware of a pupil suffering from depression and 64% knew of a youngster who was self-harming.
Forty-nine per cent were aware of children with eating disorders and 47% knew about a youngster with obsessive compulsive disorder.
What is the impact of mental health issues on pupil behaviour?
of teachers agreed it led to an inability to concentrate in class
said it meant youngsters struggled to fully participate in class
agreed it led to a pupil being isolated from other students or problems in making friends
Ms Keates said: "It is clear that teachers and school leaders are seeing many more children and young people who are exhibiting the signs of serious mental distress.
"Teachers and school leaders take very seriously their duty of care to their students and it is clear there is a great deal of concern in the profession about the gulf in the availability of expert physiological support and counselling for pupils with mental health needs."
She added: "The Prime Minister earlier this year pledged to improve mental health support for pupils.
"However, schools cannot address this issue alone and cuts to budgets and services in local authorities, health and education have all taken a heavy toll on the support available."
What contributes to mental health issues among schoolchildren?
of teachers said family problems such as ill health or a break-up had an impact
said the pressure of exams and testing
said social media played a part
said bullying was a factor
The DfE said the Government is investing £1.4 billion "to ensure all children get the help and support they need".
"No child should suffer from mental health issues," a DfE spokeswoman said.
"We are strengthening the links between schools and NHS mental health staff and later this year will publish proposals for further improving services and preventative work.
"Schools can teach about mental health in a number of ways and we have funded the PSHE Association to provide guidance for teachers on how to do this.
"We have already announced plans for every secondary school in the country to be offered mental health first aid training. We trust teachers to deliver assessment in a sensible manner that will not create stress among children."
Young Minds offers information, advice and support for young people affected by mental health issues. Find out more on their website.