Kane County Sheriff's deputies responded just before 8:30 a.m. to Kaneland High School, located at 47W326 Keslinger Road in Kaneville Township, for a reported fire in a school bathroom, the sheriff's office said.
Fire departments in the area were also called to assist.
The school's resource officer helped students and staff to safety, police said.
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The fire was put out, and the SRO was treated for minor smoke inhalation. Another staff member also received medical attention.
No students or staff was reported injured.
There was a heavy police and fire presence at the school as of about 9:30 a.m., and classes were dismissed for the day.
Police are investigating a cause of the blaze, but it was an isolated and contained incident, the sheriff's office said.
It was not a result of any mechanical issues, school officials said.
The Kaneland Community Unit School District 302 superintendent said school officials believe the fire in a boys' bathroom was set intentionally.
Kaneland school officials said students would be sent home on school buses.
Students could also be picked up on the auditorium-side, or east side, of the building.