Students at Ross Sheppard high school were sent home early Monday after carbon monoxide was detected to be at potentially dangerous levels.

Fire alarms at the school were triggered around 8 a.m. after staff reported the smell of exhaust.

"We immediately pulled the fire alarm and the fire department reacted," said school principal Ron Bradley.

Fire officials say the exhaust came from the boiler room and contained carbon monoxide. It should have been blown out of the building, but a vent was left open in the boiler room. It's not clear if the error was human or mechanical.

"Instead of the exhaust from the boilers exiting the building, they filled up the building with carbon monoxide," said Capt. Eric Takacs with Edmonton Fire Rescue.

Fire officials say it's fortunate that this happened before classes had started, so there weren't too many people inside. They say the levels of CO quickly reached dangerous levels.

"I just rode up and saw fire trucks and everyone was outside," said student Jack Edwards.

The school was evacuated, with students sent to neighbouring Westmount Mall. Eventually students were told to go home for the day.

"Sounds like a good deal actually," said student Hal Smith.

No injuries were reported. Officials say the issue has now been corrected. Classes will resume Tuesday.

The head custodian is reviewing the procedures for operating the heating system.

A spokesperson for Edmonton Public Schools says none of the division's schools have carbon monoxide detectors as they're not required by building code.

And at this point, there is no plan to install them.

With files from Bill Fortier