Prompted by soaring temperatures in “hot box” schools last summer, members of the four teaching unions are demanding the Government set a maximum temperature above which teachers are not required to work.

Last year, Pimlico school in central London was forced to send about 1,200 children home at the height of July’s 30C-plus heatwave. Schools in Swindon had to teach children in darkened corridors, and at a school in south Yorkshire holes were drilled in walls to ventilate classrooms. Workplace regulations set a minimum classroom temperature of 18C in winter, below which children must be sent home, but stipulate no maximum temperature in summer.

The Department for Education and Skills said new school buildings were not meant to exceed 28C for more than 120 hours in a school year; beyond that, schools should use their discretion over reasonable working conditions.

But the National Union of Teachers’ annual conference delegates will vote this Sunday on setting a maximum working temperature of 26C, and on fighting global warming by requiring all schools to be carbon neutral.

Dave Brinson, a teacher from Eastbourne, argued that very high temperatures could cause dizziness, fainting and heat cramps, especially in schools’

science, technology and IT labs. Steve Sinnott, the NUT general secretary, said increasing numbers of new schools with a lot of glass walls and roofs, combined with a lack of air conditioning, meant high temperatures made it very difficult to work.

“This is particularly so as our summers get hotter because of global warming,” he said.

NASUWT union members will also ask for a maximum temperature, as did members of the Association of Teachers and Lecturers this week.

“If climate change is here to stay, we will have to rethink the design of schools and even extend the length of our summer holidays,” said Phil Baker, from the Association of Teachers and Lecturers.

“We can’t have teachers and children sweltering in hot rooms, sitting there and learning nothing, just surviving.”