The shooting took place around 9 p.m. in the parking lot of Westmont High School, where a division championship game was being played between St. Francis and Serra high schools.
It sent panic through the stadium, where the gunshots could be heard.
Video posted to social media shows one team dropped to the ground, while the other ran for cover.
No one in the stadium was hurt, but two adults in the parking lot were injured, police said.
Both were taken to the hospital with what officials say are non-life-threatening injuries.
The game resumed a short time later.