No one was injured in the incident which took place at about 2:25 p.m. in the 3500-block of South Wallace Street.
Steven Dixon Jr., 22 years old has been charged with carrying a firearm at a school, child endangerment and reckless conduct.
Police said he left a bag unattended on a school bus with a gun inside.
Police said a 12-year-old student found the gun and fired it Monday.
The shot left behind a bullet hole through the bus window outside Minnie Miñoso Academy.
In a letter to parents, the principal of Minnie Miñoso Academy said, in part:
"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."
Jesse Marquez, who lives across the street, said firearms should never be left unattended.
"I don't care what type of weapon should never be left unattended. Ever. They're fatal. You can't take that back and say, 'I made a mistake.' It's too late," Marquez said.
Marquez said he believes the outcome could have been far worse.
"It's fortunate that nothing did happen. There was a broken glass. What if it was a broken head or broken heart?" he said.
Police have not said how Dixon is connected to the school or transportation provider.
Police said after the shot was fired, he grabbed the gun and bag and ran away.
Dixon is expected in court Tuesday.
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