Chicago police say the 15-year-old boy was being screened and caused a disturbance, and that is when a gun was discovered.
Charges are pending.
Chicago Public Schools issued the following statement Monday: "CPS does not tolerate the presence of any weapon of any kind on school property for the safety and security of our schools, students, staff and visitors. School officials took the appropriate steps and contacted CPD. At no time were any students in jeopardy."