At about 8:30 a.m., the student was walking in the 600 block of Chestnut Avenue when the suspect stopped his vehicle next to her, police said.
The suspect opened the door to his vehicle and had his pants unfastened. He exposed his genitals to the victim, who then ran from the area, according to police.
Once she got to school, she told her principal about the incident, and police were notified.
The suspect was last seen leaving the area toward Hillside Boulevard in a vehicle described as a bright blue, four-door sedan, possibly a Honda Civic, with chrome rims, rear tinted windows, and small decals on the windshield, according to police.
The suspect is described as being between 18 to 20 years old. He is about 5 feet 9 inches tall with an average build. He was last seen wearing a black hooded sweatshirt with a skull design on the back, blue jeans, red shoes, and a black beanie, police said.
Anyone with information about the incident is encouraged to call South San Francisco police at (650) 877-8900.