The girl is believed to have used a male student's online account to send an email to school administrators that claimed he had a gun, a bomb and to shut down the school, according to Burlingame police.
The school was put on lockdown at about 2 p.m. and the campus was evacuated and searched, police said.
No explosives were located and the lockdown was lifted, police said.
Officers detained the boy whose email account was used, and he cooperated with police.
There were no weapons in his possession and based on his statements to investigators, no arrest was made, police said.
A subsequent investigation led police to a female student who was contacted and arrested today, police said.
The girl is believed to have used the boy's email account when he logged on to a school computer and momentarily walked away from his workstation on Friday, police said.
She was arrested for making criminal threats and falsely reporting a bomb.
The suspect, whose identity was not released because she is a juvenile, was released on bail, police said.