The suspect remained on the loose after the incident at El Camino High School in Whittier around 12:30 p.m.
The incident started with a brawl between two teams in the adult league that plays at the school on weekends, witnesses said. Players say it was a coach who threw the first punch, then everyone joined in.
At least 20-30 people were involved in the fight on the field.
At one point, someone - believed to be a spectator, not a player - stabbed three people, witnesses said.
One was stabbed in the arm and two others in the upper torso, but none of the injuries were considered life-threatening. All three victims were transported to a local hospital.
The suspect hopped a fence and fled the scene as authorities arrived.