The school, located at 1 Mangini Way in Burlingame, was locked down shortly after 2 p.m. because of the threat, which was sent to school administrators, according to San Mateo Union High School District officials.
The administrators confirmed the student who allegedly sent the email was in attendance and called police. Officers arrived and located the student in a classroom and detained him, police spokesman Capt. Mike Matteucci said.
The rest of the school was evacuated while police searched the student, his backpack and locker. No evidence of a threat was found by officers or a bomb-sniffing K-9 unit, Matteucci said.
The student, a juvenile male, cooperated with investigators, who are still trying to determine if there was a credible threat or if it was a hoax.
"He claims he doesn't know anything about it," Matteucci said.
No arrests have been made and no injuries were reported during the incident, he said.
Other students were released to their parents at the school's football field after the lockdown was lifted shortly after 2:30 p.m., school district officials said.