There was an emergence response around 1:06 a.m. Thursday at Chicago Academy High School/Elementary School in 3400-block of North Austin Avenue in Portage Park, Chicago police said.
Chicago firefighters extinguished a fire on the school's third floor, police said.
The building was not occupied at the time of the fire and no injuries were reported, CPD said.
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Video shows flames coming from part of the roof and smoke inside the school.
"I actually live across the street right there so I saw it live last night at 1 a.m., senior Jordan Rivera said. "It was crazy."
Rivera watched firefighters hurry to put out the flames.
"It was scary because there was a perimeter set up around the whole school with cop cars and a bunch of firetrucks," Rivera said. "We saw the after flames and we didn't know if it was going to grow or if the school was going to come down. We didn't know what was going on."
Chicago police said no one was hurt and no one was inside at the time.
Rivera and other members of the volleyball team still showed up for their early morning practice not knowing if it would continue.
"Nothing really got communicated to us and it's just scary, the thought that the fire could've been much bigger than it was and potentially bring the building down," senior Joshua Selgado said.
Chicago Public Schools said they are not holding regular classes at the campus.
Students were told at the front door they had the option to stay or leave and most left.
But for those who did stay, CPS said they moved those students to nearby Thorp Scholastic Academy... and will return to Chicago Academy in the afternoon to be dismissed as usual.
Meanwhile, cleaning and restoration crews have been hard at work removing debris, bringing in garbage bins and large fans to air out the building from the smoke.
CPS says they will inform parents the plan for tomorrow's school day as soon as possible.
The Chicago Fire Department said the fire started on the roof in an area where workers had been earlier in the day. The cause of the fire has been ruled accidental as earlier work which caused smoldering which later erupted in open flame.
The fire caused damage mainly to the roof with minor issues on the top floor inside," CFD said.