WELSH unions are calling for action on classroom stress-related sick leave.
Teaching union NUT Cymru says more than 50,000 teaching days were lost last year as a result of workload pressures and the environment in which teachers have to work.
Now the Wales TUC has approved a motion by the union calling on the Welsh Government and local councils to work together to tackle the causes of stress-related illnesses in school.
Owen Hathway, policy officer with NUT Cymru, said: "This is a serious problem that has not been given the attention it deserves in recent years.
"We are seeing some fantastic teachers lost to the profession, either temporarily or permanently, due to stress-induced illness.
"This is not to mention those teachers who remain in post but whose work suffers as a result of draining mental health worries.
"The ongoing pressures placed on teachers, who on average are working anything up to 60-hour weeks, are unsustainable. Not only is this having a huge negative impact on pupils and teachers, but it is also costing millions."
He added: "This is a prime example of the need to invest to save."
The figures for days-off came from Freedom of Information requests to the 22 Welsh councils, which showed that during 2013 some 50,277 teaching days were lost to stress-related illness.