Police went to the school in Toronto's Scarborough neighborhood after receiving a tip about the gun about 1:45 p.m., the Toronto Star reported. The school was locked down while authorities searched the premises.
"Police searched the school and uncovered a loaded .357 revolver in a knapsack," Toronto Police Constable Tony Vella said. "We don't know why the handgun was brought into the school. That's the second part of the investigation."
Three boys and one girl were taken into custody, the newspaper said.