By Gary Eason
BBC News, at the NUT conference
Drug addiction, eating disorders and obsessive behaviours are common among teachers, a union says.
Former head teacher John Illingworth had a mental breakdown
One in three will have mental health problems at some point due to the stress of the job, the National Union of Teachers reported.
Tens of thousands suffer under the pressure of excessive work, classroom monitoring and inspections, it says.
The union's conference voted to press ministers, local authorities and school governors to improve staff well-being.
John Illingworth, a former head teacher from Nottingham, said that, after an emotional speech to last year's NUT conference about his own mental breakdown, he had begun to realise just how many faced similar problems.
A small survey conducted in his area suggested that more than seven out of 10 teachers felt their working hours were excessive.
'Reign of terror'
A few criticised the survey questionnaire as negative and wanted to say how much they enjoyed their jobs.
But 80% of the 139 who responded were anxious about Ofsted inspections and felt the increased frequency with which their lessons were monitored by school managers was adding significantly to work-related stress.
A third resorted to alcohol, smoking, unhealthy eating or "other substances" to cope.
"It's hard to imagine the emotional turmoil that drives a teacher to take their own life rather than face an Ofsted inspection," Mr Illingworth told the conference, in Harrogate.
"We are living in an educational reign of terror and we must put an end to it."
His resolution - backed unanimously by the conference - said depression and acute anxiety were a major cause of teacher absence.
Many continued to work only because of long-term medication.
"Drug addiction, eating disorders and obsessive behaviours are also common."
A secondary school teacher from North Tyneside, Magenta Stonestreet, said she worried, for example: "Is the kid that truants for five out of six weeks actually going to turn up and do my exam?"
If he did, would he fail - making her performance look bad?
Ms Stonestreet, whose lessons include psychology modules on stress and depression, said she had just returned to work after four months off through depression.
"One of the things that's quite good is that I managed to get the kids out of the classroom the other day before I locked the door and cried," she said.
Peter Laycock, 57, a former maths teacher at a secondary school in Kent, said he had found the pressures in his job kept increasing until one day "I woke up and I felt I couldn't go to school, I was in such a state".
Peter Laycock woke up one day and could not face school
Eventually he retired from teaching on health grounds. He now delivers vehicles for a rental firm.
Mr Laycock said he had enjoyed the children he had worked with but they had been difficult youngsters who did not want to learn, and it had been hard work.
His life now is stress-free and, although his head teacher had been sympathetic, he could not go back to school.
"I couldn't cope with the problems of teaching now," he said.
The union's treasurer, Ian Murch, said: "It's a scandal of huge proportion the way teachers are chewed up and spat out by our education system."
He added: "The job of the NUT shouldn't be to pick up the pieces when teachers get broken. We should be changing the system that makes it happen."
Nottingham NUT members tried to raise the issue with children's minister Beverley Hughes - who had researched mental health as a student.
She had replied that she would not meet anyone from the NUT because the union was not in the "social partnership" - the liaison forum involving government and unions.
The Department for Education and Skills acknowledged that teaching was "a rewarding but challenging job".
A spokesman said measures had been introduced to ease their burden including half a day a week out of the classroom, fewer administrative tasks and a cap on the amount of time they can be asked to cover absent colleagues.
The Teacher Support Network charity is going to work with the NUT on investigating the problem of teachers' mental illness.
Chief Executive Patrick Nash said that in the past 12 months 37% of callers to its support line had expressed feelings of stress, anxiety or depression.
"We believe that the stigmas associated with mental health issues must be challenged in order that teachers can access adequate support when experiencing difficulties at work or at home that may affect their mental and/or physical health.
"Investing in teacher wellbeing not only improves the lives of the teachers, it also has a positive impact on pupil behaviour, pupil attainment and overall school performance."