Dozens of schools in England are closing or sending pupils home early because of soaring temperatures.
The need for pupils to drink water is being re-emphasised
With poor ventilation and no air-conditioning, some schools have recorded temperatures of more than 36C (97F) in many classrooms.
A newly-designed secondary school in Milton Keynes is closing early because the amount of glass in the building is making the heat "unbearable".
A primary school in Portsmouth said pupils were close to collapsing.
The head teacher of Northern Parade Junior School decided not to open on Wednesday as a result.
The school had sent a teacher home with heat exhaustion on Tuesday.
The head teacher of Shenley Brook End School in Milton Keynes said her decision to close early was a matter of health and safety and was taken for the protection of pupils.
She said the modern design of the building was "full of glass".
"It's like sitting in your car with the windows up and the sun beating down on the windscreen," she said.
Scarcroft Primary School in York is closing at lunchtime for the rest of the week, after temperatures in some classrooms reached 32C.
Several schools in Oxfordshire have been affected.
Clanfield Primary and Bampton Church of England will close from midday on Wednesday and Berinsfield Primary, Wantage Church of England Primary, Peers School, Oxford and Iffley Meads School, Oxford are already closed.