Chicago police said a male security guard went to open the school's double door at dismissal and a suspect immediately began firing from the street in his direction.
Police said the multiple shots were fired just as students were about to be dismissed for the day.
WATCH | Chicago police update on Phillips High School shooting
"I just heard like three shots go off, and I ran back and was like, they shooting. They shooting," said Diana Taylor, student.
The security guard was struck several times, police said. Bullets also went through the school door and also struck a 14-year-old female student.
The student was rushed to Comer Children's Hospital and is in critical condition. The male security guard was taken to University of Chicago Hospital, where he is in good condition.
Several parents, including the mother of the 14-year-old who was shot, were waiting to meet their children outside the school at the time. Some apparently witnessed the shooting.
"It's ridiculous. Senseless. All this shooting for no apparent reason; it's crazy," said Bobby Taylor, father of a student.
"She could have lost her daughter. All those teens go to Phillips, could have been anyone's son or daughter with that projectile going straight through that door," said Andrew Holmes, community activist.
Det. Brendan Deenihan said the suspect fled on foot and may have escaped in a vehicle.
Deenihan said the investigation remains active and officers are currently canvassing the area for surveillance video.