Several callers reported the threats, which stated that the purported shooting would occur at the school on Thursday, said Capt. Ernie Perez of the San Bernardino County Sheriff's Department.
Deputies immediately launched an investigation, which resulted in the 16-year-old suspect being taken into custody and booked at juvenile hall, Perez said on Twitter early Thursday morning. The suspect, a minor, was not publicly identified.
"There are no other outstanding suspects and no additional threats to RCHS were identified through this investigation," the sheriff's captain wrote, adding that additional details will be released later in the day.
Authorities say they searched the teenager's home in Rialto and seized a handgun.
The Rancho Cucamonga Police Department, which is operated by the Sheriff's Department, "takes all threats of harm to our students & school staff seriously & investigate them with the highest priority," Perez tweeted.