Testing was done on 30 children and 15 were sent to hospital as a precaution on Monday.
Most were back in class on Tuesday.
Acting battalion Chief Warren Mellor said fire crews found a trace amount of carbon dioxide in the school's industrial arts wing.
But he said the amount was no different than what would normally be generated by a furnace or a clothes dryer.
Mellor said he didn't want to downplay the symptoms reported by students but said whatever they were feeling wasn't likely connected to carbon monoxide poisoning.
"Maybe somebody mentioned CO and the next thing you know they thought that was the cause and there was maybe some anxiety and it just spread," said Mellor.
However Donald Lloyd, facilities manager for the Catholic School division, isn't ruling carbon monoxide out.
"Our students were feeling ill and we believe what our students are telling us and we'll just have to continue our work in trying to identify the cause," he said.
Lloyd said a mechanical contractor was even brought in to look at various units and everything appears to be working fine, adding the school hasn't had any problems with CO in the past.
Even so, temporary CO testers remain in the building. Lloyd would also like CO detectors to become mandatory in schools.
He said at this point, he cannot comment on whether the students' symptoms could have been caused by something other than carbon monoxide.
"From my perspective as the superintendent responsible for facilities, first and foremost I'm looking at the building envelope to ensure that there's nothing within the building environment that would have caused this."