Pupils shivered in coats, hats and scarves as temperatures fell to just 1°C (34°F).
Head Rob Benzie switched off the radiators to show how the secondary school could cut its carbon footprint.
But some staff and parents slammed the "barbaric" plan.
One teacher called it "beyond stupid" and added: "It was absolutely ridiculous.
"I've never worked in such cold. I'm all for saving the planet but this was barbaric.
"Nobody could work properly and kids could not even grip a pen through their gloves." The mum of a 12-year-old at Ansford Academy in Castle Cary, Somerset, said: "She was shaking when she came home. I was absolutely furious."
A dad added: "Turning the heating off in December is just mental.
"I can't believe the kids learnt anything. I'm very angry with the school."
But Mr Benzie, 52, defended the "successful" experiment last Friday — locally the coldest day of the winter so far — and vowed to stage regular "eco-days" at the 640-pupil school.
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