(CNN) -- Six suspects accused in the gang rape of a 15-year-old girl on a California high school campus entered not guilty pleas Tuesday, police said.
A seventh person who had been arrested was released for lack of evidence, said Richmond Police Lt. Mark Gagan.
A date will be set on January 21 for a preliminary hearing, the Contra Costa County district attorney's office said.
Police say as many as 10 people were involved in the October 24 incident, which occurred in a back alley outside Richmond High School, where a homecoming dance was taking place.
Another 10 people watched the attack, which lasted more than two hours, without calling 911, police say.
The victim, who was taken to a hospital in critical condition, was released Wednesday.
Her parents have issued a statement urging the community to work to ensure that such an attack never happens again.
"If you need to express your outrage, please channel your anger through positive action," they said, according to the Rev. Jim Wheeler, pastor at First Presbyterian Church in Richmond.
CNN's Dan Simon contributed to this report.