Children as young as four are suffering from mental health problems such as panic attacks, anxiety and depression, says a teachers' union.
Almost all of the 2,000 who responded to an NASUWT survey said they had come into contact with mentally ill pupils.
Members of the teachers' union suggest schools are struggling to access enough support to deal with the issue.
The Department for Education said it was investing £1.4bn to ensure all children get the help they need.
The NASUWT teachers' union is highlighting the problem at its annual conference in Manchester this weekend and it will also warn of problems with school funding.
The survey found:
- 98% of teachers said they had come into contact with pupils who were experiencing mental health issues.
- They were most likely to be teenagers, with more than half of teachers saying they had seen issues in 14 to 16-year-olds.
- But nearly a fifth (18%) of those surveyed by the union said they had been in contact with four to seven-year-olds showing mental health issues while more than a third (35%) had seen problems in youngsters aged seven to 11.
Nine in 10 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.
Pressure of exams and testing, family problems such as ill health or a break-up and social media were all seen as having an impact on mental health.
And when asked about how it affected pupil behaviour, most teachers agreed that it led to an inability to concentrate in class and led to a pupil being isolated from other students or have problems making friends.
"An increasing number of students are having anxiety issues and panic attacks and they don't know themselves what is causing them," one NASUWT survey respondent said.
"Students generally don't know how to deal with any stresses in their lives so it tends to present itself through anger and lashing out or crying."
Another teacher said "stress, depression and insomnia due to exam revision at GCSE and A-level" were common, while another said "irrational behaviour, hurting others, trashing the classroom" were a problem.
Another said: "In the time I have been teaching (since 2007) there has definitely been a massive rise in mental health issues in the students I have taught, both secondary school and sixth form."
"It's very sad to see children so confused and mixed up at school," one teacher concluded.
Chris Keates, NASUWT general secretary, warned there was concern among teachers about a gap in the availability of experts and counselling to help children with mental health needs.
"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."
A Department for Education spokesperson said no child should suffer from mental health issues and that it was investing a record £1.4bn 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."
'Asking for money'
The National Union of Teachers is also meeting in Cardiff for its annual conference this weekend.
Speaking beforehand, the union's general secretary Kevin Courtney said: "Funding is going to be a theme that dominates the conference.
"In schools around the country, class sizes are going up. We are seeing arts, dance drama and music being cut. Vocational education is being cut.
"We are seeing schools around the country sending letters to parents asking for money on a regular basis to make up for the the gap that the government is leaving in school budgets."
The Department for Education insists that schools in England are funded at record levels and that its investment will rise as pupil numbers rise.