Investigators in Saskatoon are trying to figure out what caused St. Joseph's High School students to get sick, after they were unable to find a source of any carbon monoxide leak in the school.
Fifteen students were taken to hospital and a total of 58 students were assessed for symptoms of what appeared to have been exposure to high levels of carbon monoxide on Monday.
School officials told CBC News 12 of the 15 students have returned back to school Tuesday.
Though the children in those classroom were feeling sick, levels of CO in their blood were found to be normal or below normal.
CO levels are being monitored in the school, and temporary detectors have been installed.
An air quality expert has also been called in to examine the school.