Prairie-Hills Elementary School District 144 told families that the Prairie Hills Junior High boys basketball team was competing in a tournament at Hillcrest High School in Country Club Hills on Saturday afternoon.
The Cook County Medial Examiner confirmed there was at least one fatality related to the incident.
The shooting happened just before 1:30 p.m. in the school's parking lot, the school district letter said.
Bremen High School District 228, the district Hillcrest High School is in, also sent a letter to families, saying "a parent attending the game from a neighboring community became a victim of gun violence," calling the shooting "a tragic incident."
Witnesses told ABC7 the basketball tournament had just wrapped up when a parent was shot and killed in the parking lot.
"A kid had a great game... he's feeling good about himself and now turn that happiness and joy upside down to be at the hospital around family," local activist Stringer Harris said.
Police are not providing many details on Saturday afternoon or if anyone has been arrested, but an ABC7 crew at Hillcrest High School saw a homicide detective at the scene.
"It's sad... not only is this my former high school... I mentor here every week," Harris said. "I'm here at this school to teach about gun violence, and to see this happen here on this campus once again is sad."
The shooting comes just one year after Hillcrest High School freshman Marshawn Mitchell was shot and killed after a homecoming football game.
READ MORE | 'They took my baby': Hillcrest HS student shot to death while leaving homecoming football game
Harris said these acts of violence on campus are concerning students.
"Today I got messages from students saying they want to transfer from this school, and I'm telling them it's not a good idea," Harris said. "It's not the school that's the problem. It's not the kids that go to the school... it's always outside."
The district's letter said all Prairie Hills Junior High players, coaches and staff are safe.
A district administrator will be available over the weekend, and counseling will be available for students and staff when school resumes Tuesday, the letter added.
Further information was not immediately available.