Prosecutors say Jonathan Adam Roldan, 23, met the 15-year-old victims at Esteban Torres High School. That's where Roldan coached junior varsity volleyball and was also a campus aide.
The investigation began after one of the 15-year-old victims reported what happened to school officials. Investigators believe the relationship began in September and ended in December. Roldan is accused of having sex with the girl in his pickup truck.
During the investigation, sheriff's detectives identified a second 15-year-old student who was inappropriately touched by the suspect.
On Tuesday, Roldan pleaded no contest to one felony count of unlawful sexual intercourse with one victim and one misdemeanor count of child molestation including the second victim.
The suspect is scheduled to be sentenced on June 9. He faces three years in state prison and will be required to register as a sex offender for the rest of his life.
Roldan has been barred from having any contact with the two victims for 10 years.
The investigation is being led by the Los Angeles County Sheriff's Department's Special Victim's Bureau.