Twenty-eight people were injured when the stage gave way during a Rosary High School performance around 10:53 p.m.
Police said about 250 girls were performing in an event called the Red and Gold Rally when an apron covering the orchestra folded, and students tumbled several feet.
According to a statement from school President Peter S. Bowen posted on Servite's website, the school launched an internal investigation and has been not been able to find an apron building permit. The apron was built in the early 1980s and was not part of the original theater construction.
Servite says it's submitting plans for a new re-engineered stage apron to the City of Anaheim Planning and Building Department.
Anaheim Police Lt. Tim Schmidt said students were transported to hospitals with moderate injuries including broken bones, bruises and scrapes following the incident.
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.
"Since the incident, our primary focus has been on Rosary students, especially those who were injured, and on those audience members who witnessed the incident. We deeply appreciate your trust and patience as we moved through the investigative process to determine why this incident occurred," the statement read.
The Anaheim Fire Marshal is also investigating the incident; their inquiry is expected to take two weeks. In addition to the city's probe, the Diocese of Orange also plans to conduct an internal investigation.
CNS and The Associated Press contributed to this report.