Students at Le Quennevais School in Jersey will be sent home early this afternoon (12 July).
In a letter to parents and guardians the headteacher said the early closure is due to a lack of air conditioning in the building and they are currently "unable to open any windows".
Liberty Bus says it cannot provide an early pick-up service and parents are asked to make transport arrangements for their children.
Jersey's new Education Minister, Deputy Inna Gardiner, visited the school to see the impact of high temperatures on students and staff.
She explained: "A new cooling system was due to be installed at Easter, but the equipment did not arrive in time for the work to be completed before the summer term.
"While some localised cooling measures have been installed, and further measures will be installed in the most-affected areas during the school holidays, I will continue to review the proposed improvements to make sure they function effectively, and provide an update later this month."
Students will be allowed to continue attending school in their PE kit for the rest of term.
Sports day is being moved to Thursday morning (14 July) and the whole school awards ceremony will be on Friday morning.