A San Jose high school was evacuated and students were sent home on Friday afternoon after an unconfirmed threat to campus reached school officials, police said.
Officers responded to Oak Grove High School, 285 Blossom Hill Road, at 11:14 a.m.
They established a perimeter around the school and placed the campus on lockdown while officers investigate the threat.
Students reported on Twitter that classrooms were being evacuated one by one.
Multiple students tweeted a "Code Blue" had been called, meaning parents would not be allowed to pick up their students from campus.
Police said no suspect has been identified.
Bay City News contributed to this report.