At Pendleton, the rate of incidents was double the national average for such facilities.
Another seven sites reported nearly as high levels of sexual abuse or victimization: Victory Field Correctional Academy in Vernon, Texas; Indianapolis Juvenile Correctional Facility; Shawono Center in Grayling, Michigan; Woodland Hills Youth Development Center in Nashville, Tenn.; L.E. Rader Center in Sand Springs, Okla.; Bon Air Juvenile Correctional Center in Virginia; New Jersey Training School in Monroe Township, N.J.
At those 13 facilities, most reports of sexual victimization involved nonconsensual sex acts with another youth or serious sex acts with facility staff.
Staff sexual misconduct was higher in state-run facilities than in privately or locally operated sites, the study found, and smaller facilities tended to have fewer incidents of sexual victimization.
Nearly 80 percent of the victims said at least one incident took place in a common area, such as a classroom or office, while about half said at least one incident happened in the shower, and half of the victims said they had been victimized by staff in their room or sleeping areas.
Most of the abuse took place in the evening, between 6 p.m. and midnight, the study found.
Associated Press Writer Dena Potter in Richmond, Va. contributed to this report.