Students at William Reid School in the southwest started the week out of the classroom and into the cold.
School officials turned on the heat Monday morning and quickly noticed a carbon monoxide leak, leading to an evacuation.
Fire crews arrived at about 8:45, and found readings as high as 160 parts per million in the basement.
The kids were taken to Riverside Church for several hours while repairs were done.
Carol Henke with the Calgary Fire Department says everything went smoothly, with no injuries being reported.