Police wrote on Twitter at 12:49 p.m. about the threat directed at the school at 50 Embarcadero Road. At about 2 p.m., police tweeted again that the phoned-in threat was likely a hoax.
During the lockdown, officials told residents to expect a "heavy police presence" in the area while officers investigated the threat.
Take a look at the Palo Alto police tweets below for a look at how the day unfolded.
Click here for a look at the latest threats made at schools here in the Bay Area and across the country.
ABC7 News contributed to this report.