The defendant shot and killed a teacher, his estranged wife, and injure two kids who have been.
Then he turned the gun on himself, authorities said.
Police chief Jarrod Burguan described the event as a suspected “murder suicide” effort.
The gunman, named as 53-year old Cedric Anderson, kept his weapon hidden until he opened fire where 15 kids with special needs were being educated and had obtained entrance to the institution as a visitor, authorities said.
A nine-year old student stays in hospital in a stable state, the Los Angeles Times reported.
Mr Burguan tweeted soon following the event the defendant was “down” and there was “no additional danger”. The school was promptly evacuated.
Up to 600 pupils were transferred by bus to nearby Cajon High School, where their parents afterwards met them.
“Cops operations are continuing to ensure the region,” Mr Burguan said before on Twitter, including: “Yet, we do consider the risk is down.”