"She was raped, beaten, robbed and dehumanized by several suspects who were obviously OK enough with it to behave that way in each other's presence," Lt. Mark Gagan said. "What makes it even more disturbing is the presence of others. People came by, saw what was happening and failed to report it."
The victim remained hospitalized with non-life-threatening injuries.
Manuel Ortega, 19, was arrested at the scene and was being held on $800,000 bail for investigation of rape and robbery. He is not a student at the school.
Richmond police Sgt. David Harris said he did not know if Ortega had retained an attorney.
A 15-year-old student also was booked late Monday on one count of sexual assault, Gagan said.
Police said the girl left the dance and was walking to meet her father for a ride home when a classmate invited her to join a group drinking in the courtyard. The victim had drank a large amount of alcohol by the time the assault began, police said.
Officers received a tip about a possible assault on campus and found the girl semi-conscious near a picnic table.
Marin Trujillo, a spokesman for the West Contra Costa Unified School District, said there were four police officers and three school administrators monitoring the dance, but the assault happened away from the gym.
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