Police learned just before 8 a.m. that a student threatened to shoot up Atascadero High School, and officers went to the teen's home about 8:10 a.m.
While some students had taken to social media posting that police had found at least one gun, San Leandro police Lt. Issac Benabou said "there were no guns found".
The school was placed on lockdown; the San Leandro school district described as the decision as "strictly a precautionary move in order to systematically inspect the school grounds", KTVU reports.
All of this happened on the day that thousands of students across the Bay Area and the country were participating in March for Our Lives, a walkout to put pressure on congress for stricter gun control laws following the Parkland, Florida school shooting last month.
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A photo posted on Instagram shows the alleged graffiti threat which shows writing on a bathroom stall stating, "I'm shooting the school tomorrow (Thursday) at 1".
"We searched the school floor-by-floor, looking for anything suspicious", he said.
All other schools in the San Leandro Unified School District are open, police said. "We made a decision to work with the school and clear the grounds today - just as a precaution", said Lt. Benabou. Sheriff's officials say some students at Righetti saw the threat online and mistakenly believed it was related to their school.