Firefighters in Whitehorse are crediting alert passersby for help in preventing a bad situation from becoming much worse.   

The 911 switchboard lit up with calls about flames and smoke at the Selkirk Street School in Riverdale yesterday evening just before 7 p.m.

Deputy fire chief Kevin Lyslo said that enabled firefighters to get on the scene quickly and douse the blaze before much damage was done.

"Passersby, pedestrians and motorists passing by the school noticed smoke coming from the roof," he said.

"When we showed up there was a fair amount of smoke showing and a little bit of fire. We were able to extinguish the fire fairly quickly.

"It’s good that lots of people phoned in and didn't just assume that somebody else was making the call."

Lyslo said construction crews were working on the roof of the school earlier in the day and he believes the fire started with some sparks or smouldering wood from that work.