Classes had finished for the day, but an after-school program was underway when the fire broke at around 3:30 p.m. PT Monday.
A woman who lives across the street from the school described seeing teachers moving children away from the school.
Then she saw smoke coming from a corner of the building, and then flames erupting at the door of the nearest classroom. She said the fire grew and smoke spread over most of the school.
Class pets succumb
School District 57 superintendent Brian Pepper says the building was evacuated and all the students and staff were all outside by the time fire crews arrived. They watched, helplessly, as firefighters tried to rescue class pets.
"As you can imagine, they're quite attached to them," Pepper said. "And so it was quite worrisome and sad as both staff and students stood outside and were quite concerned about their cherished and loved pets."
Fire crews rescued rabbits and some of the class birds and a lizard, but Pepper says many pets still died.
One firefighter was injured, and parts of the school were destroyed.
The rest of the school has extensive smoke and water damage and remains closed Tuesday.
Parents are being asked to keep their children at home until alternative arrangements can be made.
Officials say it will be quite some time before students are able to return to the building.
Investigators will sift through the debris today to try to determine the cause of the fire.