About 250 girls from Rosary High School, Servite's sister Catholic school, were on the stage when it collapsed, Anaheim Police Department spokesman Lt. Tim Schmidt said.
"Early investigations suggest the front of the stage gave out due to weight," Schmidt said. Orange County fire investigators were on the scene soon after the incident, trying to determine the exact cause of the collapse.
A total of 300 to 400 students and parents were in the auditorium at the time of the collapse, officials said.
The students were performing in "Red and Gold," Rosary High's annual musical theater 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 school in Fullerton.
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