The fire broke out 45 minutes before school started on Monday. Lisa Lee and her 12-year-old daughter Johnneisha woke up to the loud roar of sirens.
"I started screaming and yelling, letting my daughter know that her school was on fire and I told her to get dressed and let's come up here and see what's going on," Lee said.
"I was actually upset because this really hasn't happened before and I was just, I was really about to cry," Johnneisha said.
Smoke was coming from the third floor of Oglesby.
"It appears like and it is under investigation that an electrical conduit pipe running along the wall from the second floor through the third floor may be the issue here," Chicago Fire Deputy District Chief Mitchell Crooker said.
Crooker said investigators are looking into whether the fire detectors worked properly.
"The system is tied into smoke alarms or heat detectors, which would essentially activate the system just as pulling an alarm," Crooker said. When asked if any of them went off, he said, "I don't believe so. That's still under investigation."
Classes have been cancelled for the day. One teacher said the fire ruined her classroom on the second floor.
"I'm really devastated. I just got new books. I got new books. We got new computers in the room," she said.
CPS said it is working closely with the Chicago Fire Department as the investigation continues. Classes will resume Tuesday.