Children as young as four are suffering from mental health issues such as panic attacks, anxiety and depression, a poll has found.
Almost all teachers (98%) say they have come into contact with pupils who are experiencing mental health issues.
These youngsters were most likely to be teenagers, with 58% of teachers saying they had seen issues in 15 to 16-year-olds and 55% in 13 and 14-year-olds.
But nearly a fifth (18%) of those polled by the NASUWT teaching union ahead of its annual conference in Manchester said they had been in contact with four to seven-year-olds showing mental health issues, and over a third (35%) had seen problems in youngsters aged seven to 11.
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.
Nine in 10 (91%) 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.
Around half (49%) were aware of children with eating disorders, and a similar proportion (47%) knew about a youngster with obsessive compulsive disorder.
The poll asked teachers about the impact of mental health issues on pupil behaviour, and 89% agreed that it led to an inability to concentrate in class, 85% said it meant youngsters struggled to fully participate in class, and 77% agreed that it led to a pupil being isolated from other students or problems in making friends.
Over four-fifths (84%) said the pressure of exams and testing was contributing to mental health issues, 71% said pressure to be good academically was having an impact, and 36% said bullying played a part.
In addition, 91% said family problems such as ill health or a break-up had an impact on mental health, while 72% said social media played a part.
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."
Last month, YoungMinds urged the Government to tackle a "mental health crisis in our classrooms".
In an open letter to Theresa May, the charity said pupils' wellbeing should be considered as important as academic achievement, and called for full funding of wellbeing initiatives, better recognition for schools that do good work on the issue, and specific mental health training for teachers.
:: The NASUWT poll questioned 2,051 members in March.
A Department for Education spokeswoman said: "No child should suffer from mental health issues and we are investing a record £1.4 billion to ensure all children get the help and support they need.
"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."