Children are being taught in classrooms that are too hot, teachers have warned.
The NASUWT said that excessive temperatures hamper pupils' ability to learn and teachers' ability to teach.
A poll published by the union has found that 93.7% of teachers have experienced temperatures in excess of 24C, while a third (33%) have faced temperatures of over 30C.
The survey is based on more than 1,000 monitoring forms by teachers, containing over 19,000 temperature recordings taken during four weeks last summer.
It also found that more than three quarters (77.2%) of teachers have faced temperatures of 24C on at least a quarter of the days during the survey period.
Almost half of those who took part (44%) said that there is some impact on a pupil's ability to learn when a classroom is at between 24C and 26C, while 30% said the impact was considerable or very considerable.
And 50% said there is some impact on a teacher's ability to teach at 24-26C.
The findings were published as the NASUWT prepared to debate a motion on health and safety in schools at its annual conference in Birmingham.
The motion warns that there is "compelling national and international evidence that confirms the prevalence of serious health and safety risks in schools and colleges, including exposure to work-related stressors, excessive working hours, bullying and harassment, asbestos and excessive temperatures".
It calls for the union to continue campaigning for legislation to protect the health and wellbeing of teachers, support staff and pupils, and the enforcement of statutory health and safety provisions.
There should also be published guidance on managing health and safety risks, it says.
NASUWT general secretary Chris Keates said: "NASUWT research has shown time and time again that teachers are facing serious health and safety risks in schools as a result of high levels of stress, school buildings which are outdated and not fit for purpose, the presence of asbestos and excessive classroom temperatures."
A Department for Education spokesman said: "All schools have to comply with workplace regulations on temperatures. There is therefore no need for further unnecessary regulation on this."