Work has begun to stop classrooms from overheating in six newly built schools in Exeter.
The schools, which were constructed under the private finance initiative, do not have air conditioning systems.
Students say the schools are too warm to study in, with some reaching temperatures of 33C (91F).
The company that provided the buildings, Carillion, says its first priority is to change the ground floor windows so that they can open.
It will also install skylights in some of the schools.
Those affected are St Luke's Science and Sports College, St Peter's, St James, West Exe Technology College, Isca College of Media Arts and Wynstream Primary School.