Police said a man left a bag unattended outside of Minnie Miñoso Academy near West 35th and South Wallace streets in the Bridgeport neighborhood just before 2:30 p.m.
A 12-year-old boy found the bag, took a firearm from inside, and discharged one round, shattering a bus window, police said.
Police said the man then took the gun and bag before fleeing. Police have not identified the man or said how he might be connected to the school or transportation provider.
"Kind of looked out the window and saw more police activity than normal around here," said Miguel Lozano, who works in the area. "I think it poses a lot of questions, like, how and why."
There were no reports of injuries, and area detectives are investigating.
The school's principal issued a note to parents and guardians, saying, "The safety of your children is always our top priority, and we are writing to inform you of an incident impacting our school. Today at dismissal, there was a shot fired from inside a school bus transporting students from our school. The shot hit a window on the bus. Thankfully, no one was injured, and all students are safe. The Chicago Police Department (CPD) and the CPS Office of Safety and Security were immediately notified. CPS is working with CPD to investigate this situation further. While I cannot provide further details for privacy reasons, I want to assure you that the gun did not enter school grounds and everyone is safe. Please know that we take situations like this one extremely seriously. As a reminder, if you are ever made aware of a potential safety concern related to our school, I ask that you notify us immediately so we can ensure the concern is properly addressed. We will continue striving to provide all students with a safe learning environment. If you have any questions or concerns, please contact me directly."
Authorities did not immediately provide additional information.
INTERACTIVE SAFETY TRACKER Track crime and safety in your neighborhood