Children were playing on the Stony Mountain School playground before classes Wednesday morning when at least 35 of them and two teachers began complaining of burning eyes, runny noses and coughing, the Winnipeg Free Press reported.
Parents and teachers herded the children into the school building, which was soon surrounded by the Royal Canadian Mounted Police, paramedics, fire personnel and health and safety officials, the newspaper said.
It was soon determined the source of the irritant was the federal penitentiary about 500 yards away, where media spokesman Chris McLauchlan said staff had been doing twice-yearly training with tear gas.
"We've been doing this for more than 10 years and we've never had an incident before," he said.
The Free Press said there was "little or no wind" at the time.
The children and teachers were examined at the school and sent home for the day with no lasting injuries, CTV News said.