Police, firefighters and medics responded to a call shortly before 11 p.m. PST Saturday after the stage gave way at Servite High School, according to the Anaheim Fire Department.
The dispatch supervisory office told The Associated Press that 30-40 people were transported to hospitals.
Most of the injuries are minor and none is life threatening.
The office said it's not immediately clear if the injured are students.
Anaheim police Lt. Tim Schmidt told local news organizations that the injuries included broken bones, bruises and scrapes.
He told the Los Angeles Time that about 250 girls from sister Catholic school Rosary High School were on the stage at the time.
"Early investigations suggest the front of the stage gave out due to weight," Schmidt said.
The newspaper says 300 to 400 students and parents were in the auditorium at the time of the collapse.
The students were performing in "Red and Gold," Rosary High's annual musical theatre challenge, in which teams prepare for six weeks to put on choral, dance, drama and other performances, according to the school's website.
Servite High School is an all-boys Catholic School in Anaheim. Rosary is an all-girls sister school in Fullerton.