A security was temporary absent when a high school student past school metal detector carrying a handgun, a surveillance tape showed.
The gun discharged at Chicago Vocational Career Academy, wounding the 15-year-old student and a 14-year-old classmate.
The tape showed that the student bypassed security on April 10 when the guard "momentarily stepped away" to escort another student to the attendance office, according to Chicago Public Schools spokesman Malon Edwards.
"He should never have left the metal detector unmanned," Edwards said.
Edwards declined to give the name of the guard or say if the guard would be disciplined.
Demond Gore brought the 9 mm handgun to school, officials said. He was sitting in science class when he took out the handgun and pulled the trigger. A bullet struck Gore in the inner left thigh, traveled through his leg and struck Jodale Woodfork, 14, also in the leg.
Both students were hospitalized.
Gore was charged with aggravated unlawful use of a firearm within 1,000 feet (300 meters) of a school and faces a sentence ranging from probation to three years in prison.