Schools are set to face further disruption on Friday as the ongoing heatwave forces yet more closures.
At least 13 schools closed early or were totally shut on Thursday, according to Essex County Council.
It comes as most of Essex remains under a red weather warning, with the Met Office forecasting temperatures could hit 38C (96.8F).
In a letter to Education Secretary Bridget Phillipson, the National Education Union (NEU) said the heat was “significantly” impacting children’s studies.
There were six emergency school closures in Essex on Thursday, including Boreham Primary, De la Salle in Basildon and The Belsteads in Little Waltham.
Brentwood’s Endeavour Co-operative Academy also shut, saying its air conditioning units were “working beyond capacity, resulting in poor ventilation”.
“We have serious concerns [about] the health, safety, and welfare of our learners,” it said.
