According to a post on the school's Facebook page, law enforcement officials locked down the campus after "an external source called 911 at 9:45 a.m."
Officers responded to the school at 28755 El Toro Rd, the Riverside County Sheriff's Department said on Twitter, adding that it was "NOT an active shooter situation."
The campus was being searched as a precaution, the sheriff's department said.
All students will remain locked in classrooms until authorities complete the sweep, according to the school's statement.
"Everyone is safe and so far, there is no validity to the 911 call," the statement added.
Shortly before 12:30 p.m., the sheriff's department announced the campus was cleared and the lockdown was lifted.
"No threat to students at Temescal Canyon High located," the agency said on Twitter. "School resuming regular schedule."