Police said they discovered the kittens March 7 at the boy's Hacienda Heights home after he told a staff member at Wilson High School of the incident, the Los Angeles Times reported Friday.
"Apparently the boy confessed to a school staff member that he had killed three kittens earlier in the morning by beating them with a baseball bat and disposing of the bodies in the trash at his home," said Marcia Mayeda, director of animal care at Los Angeles County Animal Care and Control.
Animal control officers checked the 2-week-old felines for signs of life but determined the animals were dead. The kittens had severe fractures to the skull and head trauma.
"Cruelty towards animals is usually a precursor to other human-directed violence later in life. It is crucial to intervene in these types of cases early on to avoid the escalation of violent behavior," Mayeda said.
The report did not indicate whether the teen gave a reason for the beatings.