Ambulance spokesman Troy Davies said many of the students at St. Joseph's High School in Saskatoon had headaches and were being tested as a precaution.
SaskEnergy spokesman Dave Burdeniuk said the leak didn't appear to be a major problem as carbon monoxide readings were "almost undetectable."
"The readings are very, very low, almost to the point where they can't be picked up by detection equipment," he said.
Saskatoon Fire and Protective Services said workers measured about five parts per million of carbon monoxide, which is typically too low a reading to cause a problem.
Assistant fire chief Bill Coffin said it's not usually a concern until measurements are double that.
But Burdeniuk said students were still feeling the effects from the gas, which included headaches, dizziness and nausea.
He said it was too early to know the exact cause, but he suggested there could be a problem with the school's boiler.
Carbon monoxide is an odourless, colourless gas found in fumes produced by cars and trucks, small gasoline engines, stoves, lanterns, burning charcoal and wood and gas ranges and heating systems.