Three pupils collapsed from heat exhaustion earlier this month because of soaring temperatures inside three of Bradford's new flagship secondary schools, according to teachers unions.
Tong High, Buttershaw Business and Enterprise College and Titus Salt High School were the first schools to be completed under the region's pound;400 million Building Schools for the Future programme last year, and are said to boast state-of-the-art facilities.
But according to the Association of Teachers and Lecturers, a pupil from each school was sent home after passing out due to excessive heat inside the new buildings during the mini heatwave that hit the UK at the end of June.
The union added that at the peak of the heatwave a fortnight ago, temperatures reached a staggering 38C in the classroom.
Stuart Herdson, Bradford branch secretary of the ATL, told The TES: "The recommended air temperature inside a classroom is between 18 and 26 degrees, but we had classrooms that were 38 degrees in the shade.
"We have been told that a pupil had collapsed in each of the schools, and even a number of staff went home feeling quite ill."
He added: "These schools are brand new so you wouldn't expect this sort of thing to happen. And apparently the opposite effect can happen in the winter and the classrooms are freezing cold."
According to the NUT, the main issue is the large amount of glass in the buildings. Rather than using air conditioning units to cool the air, the schools have mechanical ventilation that controls the internal temperature by sucking cooler air from outside.
But the union said that either the ventilation systems were not working or school staff had not been taught how to use them properly because many of the classrooms are "unbearably hot".
The NUT is calling for the school to be closed if classroom temperatures exceed 26C when pupils return after the summer holidays.
Miriam Murch, health and safety adviser to the Bradford branch of the NUT, said: "When we saw the plans, we said there was a lot of glass and asked them how they were going to get the hot air out. Their response was `this is 2009, we have these systems' as if what we were asking was stupid.
"One of the schools hired industrial cooling units but they make teaching almost impossible because they are so noisy, and they only have half a dozen of them."
Lyn O'Reilly, Tong High School head, said: "I think people suffered in all buildings during the heatwave. But temperatures went beyond what you would expect, especially during the examination period. I know schools in Leeds also had some trouble.
"The problem was with the ventilation and in particular areas with lots of glazing. We didn't have to close classrooms, but where it became too uncomfortable we moved the pupils. We are working hard to sort the problem and have taken measures with cooling units in the meantime."
Bradford Council deputy director of services to children and young people Cindy Peek said: "Some schools have been having difficulty in keeping rooms at a comfortable temperature - but this is not unique to educational settings given the extreme temperatures that have been experienced across the country.
She added: "Some schools have been given extra support to help them cope and they were offered mobile air-conditioning units to help keep classrooms cool."
Tong and Titus High were both shortlisted in the outstanding new secondary school category in this year's TES Schools Awards.
The government agency overseeing the pound;55 billion Building Schools for the Future programme has been accused of "bullying tactics" in a think-tank report this week.
Policy Exchange quotes local authorities describing Partnerships for Schools (PfS), the organisation behind BSF, as "marching round the country in their jackboots, telling local authorities what to do".
Since the publication of the report, Policy Exchange claimed that PfS has been "putting pressure" on people who gave evidence for the study.
Anna Fazackerley, head of education at Policy Exchange and report co- author, said: "We have been contacted by a number of people who have been very upset after receiving a phone call from PfS accusing them of speaking out of line and putting pressure on them to retract their views.
"They are frightened about what will happen to their BSF budgets. Some have been in a terrible state.
"It is outrageous that a public body should attempt to suppress debate in such an underhand manner."
A PfS spokesperson said: "This is a laughable accusation and only slightly less laughable than the accuracy and content of the report."