Point Loma High School was placed on lockdown on Wednesday morning for more than two hours after an unsubstantiated email threat.
School officials said they received an email threatening the school and the lockdown was implemented at about 8:15 a.m.
Officials declined to comment on details of the threat.
Police were sweeping the school at 9 a.m., as students were being told to shelter in place.
The school's principal sent out voicemail messages to parents, alerting them to what was described as an "unsubstantiated threat."
He said standard protocol calls for police to sweep the school and that students were safe in their classrooms with teachers.
By 10:30 a.m., students were given the "all clear" and the lockdown was lifted after police found nothing unusual on school grounds.
Parents outside the school expressed relief that everything turned out OK.
"They’re doing what they’re supposed to do by keeping the children in and keeping them safe," said parent Amanda Trompas.