"Can you imagine trying to learn under 35-degree conditions?" asked Miller. "I teach phys ed -- good luck with that, too."
But education and health officials say there do not appear to be any rules governing when it's too warm in school -- though there are lots of rules about severe cold conditions.
And they can't recall anyone raising safety concerns or filing workplace health and safety concerns.
"I think it's ridiculous," Miller said. "We're here six and one-half hours. We need some air conditioning, we need an environment conducive to learning."
Miller said he hasn't raised a formal complaint before, but after 30 years of teaching he's close enough to retirement that he's willing to speak out.
Miller teaches at Hampstead School in River East Transcona, where he says the temperature reached the high 30s and low 40s this week, when the humidity was taken into account.
"I think it's a major concern. Our staff is just dragging," Miller said.
"There's just no rules on that. Most people use common sense," said Heidi Graham, spokeswoman for the Winnipeg Regional Health Authority.
School divisions have rules for pulling buses off the road, cancelling school or calling off outdoor recess in severe cold, but not for the opposite end of the thermometer.
"I'm not sure there is any standard," Manitoba Association of School Trustees executive director Carolyn Duhamel said. "The temperature goes up quite considerably in some of those buildings."