Environment Canada is trying to determine whether a tornado was responsible for ripping the roof off an elementary school in Hamilton, Ont., on Wednesday, slightly injuring two students.

The children were among a group practising volleyball in the gym of the Lawfield Public School at about 4 p.m. when a bout of severe weather hit.

"I just heard a big gust of wind and the roof just collapsed," said Matt Theoret, one of the students in the gym. "The windows blew in... We all ran."

Two students who were hit by falling debris were taken to hospital for treatment.

"This emergency scene could have been much worse and a lot more tragic just due to the fact that if it had happened a half hour earlier, the elementary school would have been filled with children or with children leaving," said Bob Simpson of Hamilton's EMS.

A number of children still doing after-school activities at Lawfield were evacuated to a nearby arena, but then that building's roof began peeling off.

School officials have closed Lawfield until further notice.

Eyewitnesses reported black skies and funnel-shaped clouds just before the bout of severe weather hit, downing trees and power lines.

Environment Canada has yet to confirm a tornado was responsible for the damage, since an on-site assessment must be done. That will happen Thursday, when weather experts have a chance to examine the pattern of debris and the extent of the damage.

However, Hamilton is in a part of southern Ontario where tornadoes have hit in the past.

Houses damaged as well

A large tree fell in front of Steve Burkholder's house and the wind flipped over his camper trailer, which had been parked nearby.

Burkholder was inside with his daughter at the time.

"I just felt the windows kind of go whoosh," he said, describing how he looked outside to see his trailer sitting on its roof on his front lawn.

The winds took a toll on Hazel Clarke's house "We were all just relaxing and then we heard thunder and lightning and the door banged," she said. "Then we heard it again and all of a sudden the roof caved in. The whole of the roof of the house is gone."

About 5,000 homes were without power in the wake of the storm, but electricity had been restored to most of them by Thursday morning.