Teachers called yesterday for schools to close if classrooms become too hot in the summer.
Poorly-designed buildings can send heat levels soaring, leading to pupils fainting and having problems concentrating in lessons, the National Union of Teachers said.
Delegates at the union's annual conference in Harrogate backed a motion urging staff to abandon their classrooms when temperatures are hotter than 26C (79F).
Brandishing a fan, Pat Sexton, a maths teacher from East Sussex, told the conference she did not want all her pupils fanning themselves during lessons because they were too hot.
"There are times when the temperatures are well over 80F and that seems to me to be the time when we are less effective," she said.
"I am hot and sweaty, they are hot and sweaty and neither of us are in the condition to teach or learn."
During the winter, extreme cold weather forces schools to close.
"So in the summer if temperatures soar then it may be necessary to disrupt children's schooling."
She suggested that lessons in classrooms which become particularly hot - such as science labs and computer rooms - should take place "first thing in the morning when temperatures are cooler".
Dave Brinson, a delegate from Eastbourne, told the conference: "The problem is only going to get worse.
"Climate change is a long-term problem that is going to require action across the world."
New schools - such as privately-sponsored city academies - built with large glass windows may "look wonderful" and win architecture prizes but they are "completely unfit for the purpose".
The motion called for ministers to draw up new regulations requiring schools to make sure temperatures do not rise above 26C.
It said: "Teachers should not and cannot be expected to work in any classroom or other internal teaching space where the temperature exceeds 26C for anything other than very short periods."
NUT executive member Pete Bevis told the conference: "Frequently, during periods of hot weather our members have had to endure working conditions that are unacceptable.
"Other speakers have referred to the hot, sweaty, bothered and fractious individuals - and then there are the pupils as well."
Jerry Glazier, from the union's executive, said many classrooms built in the 1960s and '70s "become greenhouses especially when they are facing south with windows often secured to only open a few millimetres for safety reasons".
A DfES spokesman said: "Comfortable working conditions are vital to effective teaching and learning and we expect schools to use their discretion in applying all relevant regulations - rather than laying down inflexible guidelines that may deny young people education.
"Our unprecedented capital investment in schools is precisely addressing the legacy of poorly designed and maintained buildings.
"Every secondary school and half of all primaries are being rebuilt and refurbished to the highest standards."
Officials said statutory building regulations already state that all new schools should be designed so that temperatures do not exceed 28C for more than 120 hours in a school year.Reuse content