The incident has left the community of Woodlake in shock.
Woodlake School District officials say the 17-year-old suspect was at Castle Rock Elementary when it happened.
He was completing community service hours as part of a graduation requirement.
Woodlake Unified Schools Superintendent Drew Sorensen says the District was notified the morning after the incident.
"The only way that you're going to find out about these things is if somebody's willing to talk. Too many times the stigma has been you don't talk because somebody blames themselves for something. That's not true. We know that. We've all learned that."
Woodlake Police say it happened last week, on the 27th, when they were told of a sexual assault that happened at Castle Rock.
Officials say it was an isolated incident, and are grateful they found out as soon as they did.
Sorensen says that could have something to do with the school's buddy system.
"If they think there's something that isn't right, they're going to talk to their principal, or talk to the learning director."
It's not known how the suspect was able to commit the crime, but district officials say there was plenty of supervision that day.
But Superintendent Sorensen says the District may have to make some changes.
"It throws you back a bit to try to figure out, okay, there have to be some more rules and regulations."
Woodlake Police have been investigating the incident, and have now submitted it to the Tulare District Attorney's Office.
Woodlake Unified School District is fully cooperating with law enforcement, and plans to conduct its own investigation.