A 19-year-old former student and a 15-year-old current student have been arrested, but more arrests are expected soon, reports CBS News correspondent Sandra Hughes.
"Multiple offenders, multiple people raping over prolonged period," said Lt. Mark Gagan with the Richmond police.
What makes this crime so shocking is that police say at least 20 people were involved in the rape or stood and watched the crime without going for help.
"These are witnesses that are encouraging and allowing behavior to continue," Gagan said.
For those who watched and did nothing, it may be morally reprehensible, but it's not illegal.
"Although this is a very difficult crime to hear about, the fact is California law does not impose an affirmative obligation on anyone to do anything when you are watching a crime," said Trent Copeland, a legal analyst.
Richmond High School has a 69 percent truancy rate and is in a school district that had 19 student murders last year. Fights at Richmond have been posted on YouTube, and a van was set on fire on the football field's new artificial turf. An investigation by CBS station KPIX-TV last year found only a handful of the 16 security cameras in the school were working.
On Friday, there were four Richmond police officers on duty at the dance, but the security guards patrolling the grounds were released at 9 p.m., a half-hour before the victim left the dance.
"Dance was successful event and safe for the students that were there," said Marin Trujillo, the West Contra Costa Unified School District spokesman. "This dance itself was a successful event."
Police are now searching for cell phone video that onlookers may have recorded. Wednesday, it's expected that the suspects will be in court for arraignment.