The shooting, which occurred about 6 p.m. Friday in the gymnasium parking lot at Washington Preparatory High School in Athens, was "absolutely gang-related," said Los Angeles County Sheriff's Det. Steve Pratt, who is investigating the case.
The names of the suspects were not immediately released and no other details were available.
"We still have a lot of leads to follow," Pratt said.
Both victims, who were shot multiple times, were in stable condition Saturday at Harbor-UCLA Medical Center near Torrance, a nursing supervisor said.
Neither the girl nor the 19-year-old man, who graduated from the school last year and played on the football team, were identified.
Los Angeles Unified School District officials said additional security will be provided Monday at the school on South Denker Avenue at West 108th Street.
Witnesses who were back at the school Saturday said gunfire broke out minutes after Washington Prep defeated University High, a Westside school.
"People started screaming and running," said Morgan Chancelien, 16, who attends Washington Prep.
Extra security personnel will be at the school throughout next week, said Susan Cox, a school district spokeswoman.
Additional security also will be provided at Audubon Middle School, where the 12-year-old is enrolled.
Cox said crisis counselors will be sent to both schools to help students and staff.
"They'll be on hand to lend an ear or provide any assistance," she said.
After the shootings, at least one assailant was seen running south from the school, Cox said.
School police and Los Angeles County sheriff's deputies shut down several blocks around the campus as they searched neighborhood apartments, Cox said.
The search was called off about 9 p.m. Friday and residents were allowed to return to their homes.
Lt. Hector Rodriguez of the Los Angeles School Police Department said shell casings found at the scene might have come from two guns.
Security had already been heightened for Friday's game because of rising gang tensions in the neighborhood, said a district employee familiar with the investigation who did not want to be named because he was not authorized to speak.
There were reports earlier in the day of suspects with guns near the campus, so the school had extra officers on hand, he said. A student was stabbed to death in the same parking lot in May, he said.
Times staff writer Robert Lopez contributed to this report.