Wearing bulletproof vests, three teen suspects have appeared for the first time in a US court on charges of gang raping a 15-year-old girl outside a high school dance while as many as two dozen people watched without calling police.

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 on Thursday.

Peter's aunt, Monica Peter, said before the hearing that her nephew had told her he was only a bystander and did not participate 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. Morales had a black eye from a separate incident, authorities said.

Morales' lawyer, Ernie Castillo, said he was still learning details about the case.

"I don't know what limited role, if any, my client played," Castillo said. "This was a tragic, horrible incident that happened to this young woman."

Reports have indicated onlookers laughed and joked during the attack on Saturday night. Police would not comment on rumours the episode had been recorded on mobile 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 and could be sentenced to life in prison, if convicted.

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

Another suspect, 19-year-old Manuel Ortega, 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 $US1.2 million ($A1.34 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 mobile 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 on Wednesday.

Police are still looking for more suspects and believe as many as 10 people may have attacked the girl for more than two hours in a dimly lit area. Terry Collins reporting for Sudney Morning Herald.

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