Defendant Cody Ray Smith, 15, pleaded not guilty, while Ari Abdallah Morales, 16, and Marcelles James Peter, 17, did not enter pleas during their arraignment in Contra Costa County Superior Court.

Peter's aunt Monica Peter said before the hearing that her nephew had told her he was only a bystander and had not participated in the attack.

Asked why he didn't do anything to stop it, she said he feared "he would get his ass kicked."

The defendants wore the vests for protection, but sheriff's spokesman Jimmy Lee could not explain further.

Reports have indicated that onlookers laughed and joked during the attack. Police would not comment on rumors the episode had been recorded on cell phones.

Smith, Morales, and Peter were each charged with rape in concert and sexual penetration with a foreign object. They were being held without bail.

They were charged as adults because of the severity of the crime, which showed "callousness and viciousness," prosecutor Dara Cashman said.

"It's just very cold," she said after the hearing.

Another suspect, Manuel Ortega, 19, was arraigned separately but did not enter a plea. He is charged with rape, robbery and assault causing great bodily injury. He is being held on $1.2 million bail.

Salvador Rodriguez, 21, also remained jailed but has not been charged.

Police said the victim left the homecoming dance at Richmond High School and was walking to meet her father for a ride home when a classmate invited her to join a group drinking in the school courtyard.

The girl had consumed a large amount of alcohol by the time the assault began, police said. The girl's father tried to call her cell phone but no one answered.

Police received a tip about a possible assault on campus from a former student, who heard two males bragging about it. Officers found the girl semiconscious and naked from the waist down near a picnic table.

She was released from a hospital Wednesday.

Police are still looking for more suspects in the Saturday night attack and believe as many as 10 people ranging from 15 to the mid-20s may have attacked the girl for more than two hours in a dimly lit area.

A $20,000 reward is being offered for information.