The school was placed on lockdown while officers investigated the threat.
Since the Florida shooting, students have been leading the charge to demand lawmakers pass new laws to prevent future school shootings.
That same user had earlier tweeted: "I am a student at San Leandro high school and I'm tweeting for the students and I can confirm we are all now safe and locked inside of our classrooms".
"The graffiti was written sometime between the start of school and 9:30 a.m.", he said. No weapons were found. Police say everyone was safe, and students were dismissed at their scheduled time.
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Police are working with district and school staff to ensure safety on the campus and to systematically inspect the school grounds. The graffiti threatened on-campus violence for Thursday.
District officials notified all families in the San Leandro Unified School District of the lockdown.
While the students were not allowed outside to participate in the walkout, some students posed with their banners calling for change and posted pictures on social media.