The bug spraying on Saturday was done by an outside contractor using a product that wasn't approved by the board, said Norah Marsh, the associate director of education at the Ottawa Carleton District School Board.
The school had been sprayed during the March break to deal with a cockroach problem, but a second spraying was needed, Marsh said.
When students and staff returned to the elementary school following the break, there was an odour from the spray, but no one showed symptoms of illness, said Marsh.
30 kids showed symptoms after spraying
After the second spraying, about 30 children and some staff began to show reactions including watery eyes, itchy ears and nausea, and some children went home sick. Parents were informed on Wednesday, Marsh said.
Ottawa Public Health, the Ontario Ministry of Labour, Health Canada and the Ontario Ministry of Environment are all investigating.
Marsh said no one advised the board to close the school, but the decision was made to allow more time to clean and ventilate the building.
Peter Giuliani, president of the local chapter of the Elementary Teachers' Federation of Ontario, said the board did the right thing closing the school, but added it might have been wiser to do it sooner before more people showed symptoms.
"I hope we learn from it … next time act faster," he said.
Marsh said staff members expected the odour to dissipate on Monday, and once the smell persisted they realized an outdoor pesticide had been used.
The school closed its doors after classes were done Thursday. All community events and an international language program scheduled for Saturday have been cancelled.
About 450 students from kindergarten to Grade 6 attend the school at 2605 Alta Vista Dr.