Stress was responsible for a third of teachers' sick leave last year in England and Wales, claims a survey.
Teachers are meant to benefit from a workload reduction agreement
The survey, from the Schools Advisory Service insurance firm, said teachers' absenteeism had risen by 11% in the past five years.
It estimated that the annual cost to schools from staff being missing because of work-related stress, anxiety and depression was £19m.
The total number of days missed because of stress was 213,300, the survey said.
The survey, from an insurer which provides cover for staff absenteeism, said increases in staff absences were being driven by rising levels of stress.
Les Marshall, the firm's director, said staff with stress-related problems were typically away from work for six school weeks.
Such lengthy absences could put much more pressure on a school than a few days off for colds and minor illnesses.
Mr Marshall said efforts to reduce teachers' workload - and to improve their work-life balance - had so far not shown any signs of making an impact.
The findings were based on the insurance claims made by schools - and Mr Marshall said schools were identifying stress as the main reason for a teacher being off sick, rather than only as a contributory factor to other ailments.
The annual total for days lost to sickness was 639,000 - representing an average of 11 days per teacher who was on sick leave.
A spokesman for the National Association of Head Teachers said work-related stress was one of the main causes of teachers' sick leave - and he urged early intervention.
"The early identification of symptoms and support for staff is important, as prevention is always better than cure," said the NAHT's John Randall.
The Department for Education and Skills responded to the survey by saying that the "number of days lost through sickness compares favourably with other occupations in both the public and private sectors".
"We nevertheless have a range of measures to ensure that teacher health and well-being is taken seriously, which include a focus on tackling sources of stress and excessive workload," said a DfES spokesperson.
"Programmes to improve pupil behaviour are providing positive working environments for teachers while our reforms to the school workforce are cutting bureaucracy and reducing the burden of work for teachers."