Judge Gregory Caskey of Contra Costa County Superior Court freed the youngest defendant, 16-year-old Cody Smith of San Pablo, because an investigator had interrogated Smith in violation of his Miranda rights, the San Francisco Chronicle reported.
The judge also reduced charges against Elvis Torrentes, 23, of Richmond, who said he had only consensual contact with the victim before leaving the scene.
The other five defendants still face up to life in prison if convicted of the Oct. 24, 2009, crime at Richmond High School.
The judge made his decision after four weeks of gruesome testimony in preliminary hearings.
At issue was whether the victim, a Richmond High student with an extremely high alcohol level, was forcibly raped or just set upon by opportunists; and whether witnesses, most of them also also accused, were credible.
The girl has told police she has no memory of being raped.
Most defendants will have to be tried separately because of protections against being implicated by co-defendants who can't be cross-examined, the newspaper said.