Two adults are dead in a San Bernardino North Park Elementary School classroom in what is believed to be murder-suicide, San Bernardino Police Chief Jarrod Burguan tweeted.
Authorities believe the suspect is down and there is no further threat, Burguan said.
California authorities responded to a report of a shooting at the school.
Two students were transported to a hospital.
The other students in the school were being evacuated and taken to a nearby school.
Parents of students attending the La Fillette Roman Catholic School have staged a protest for the second consecutive day, calling on the Ministry of Education to provide them with a new school building.
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A new state-of-the-art school had been promised to the community by the last Government, at an estimated cost of $17.4 million.
It was due to be completed by Easter this year.