No major injuries were reported. Authorities believe the collapse was due to weight.
Authorities responded to the school on the 1900 block of West La Palma Avenue around 10:53 p.m. There were between 300-400 people in attendance, and the event was sold out.
About 250 girls from Rosary High School were performing in an event called the Red and Gold Rally when the stage collapsed, police said. According to witness Lupe Martinez, the stage folded in the middle and the girls were dropped 10 feet into the pit.
Witnesses said no one could believe what had happened.
"They were on their final performance so everyone was jumping around like crazy, and then all of a sudden the very front row just completely fell under. We were all in shock," said Stephanie Stevenson.
"We were all jumping, having fun. We've done it five times before, and then all of a sudden, the girls in front of me just disappeared," said Rosary High School student Sabrina Lee. "They were below, like the stage fell, and I don't know what happened, all I knew is I turned around and I ran.
Anaheim Police Lt. Tim Schmidt said students were transported to hospitals with moderate injuries including broken bones, bruises and scrapes.
Rosary is Servite's sister Catholic school in Fullerton. The Red and Gold Rally is Rosary's musical theater challenge in which teams prepare for six weeks to put on choral, dance and drama performances.
Inspectors will double check the school's stage permit to determine if it's up to date. They will also review whether the permit included a weight restriction or a limit on the number of people the stage could hold. The investigation is expected to take two weeks.
CNS contributed to this report.