The shooting happened at the end of the school day, around 3:30 pm, at Wendell Phillips Academy High School, Chicago Police Chief of Detectives Brian Deenihan said during a news conference.
The guard opened the school’s door and “immediately, there was an offender in the street area who started shooting in the security guard’s direction,” Deenihan said.
The guard was shot several times, and the bullets went through an unopened door, striking the teenager, Deenihan said. She was brought to a hospital and was in critical condition Tuesday.
The security guard, 45, was also taken to a hospital and was in fair condition, police said.
Investigators don’t know whether the shooting was targeted. There have been no arrests, police added.
The shooting comes after dozens of people were shot in the city over the weekend.
Forty people were shot in 31 shootings in Chicago over the holiday weekend, according to the Chicago Police Department’s major incident notifications. Four victims were killed.