The lunch period attack left one student at Mattoon High School wounded and in stable condition at a hospital. The student-suspect is in custody.
9-1-1 audio indicates a woman hiding in a staff bathroom called police and reported hearing several shots.
Mattoon High School is about 180 miles south of Chicago, between Champaign and Effingham.
The gunman was disarmed by a teacher before firing additional shots or wounding anyone else, according to investigators.
Dozens of Mattoon officers, deputies from the Coles County Sheriff Dept. and Illinois State Police troopers responded to the active shooter call.
When police arrived the school was already being evacuated. Officers then cleared the building, room by room.
Authorities have not released the suspect's name or a possible motive for the school shooting.