The message -- in which someone threatened a campus shooting on Thursday -- was discovered on the wall of the bathroom stall on Tuesday, Pleasanton Unified School District spokeswoman Nicole Steward said.
Faculty and staff were briefed by police about the threat at a staff meeting this morning, and have been instructed to talk to each of their classes about the incident, she said.
In a message sent out to parents Tuesday, Principal John Dwyer said the graffiti has been photographed and has made the rounds on social media, "so there are rumors circulating."
Steward said today's school attendance does not seem to have been affected by the threat, though she said it has prompted a handful of parents to notify the school that they will keep their children home on Thursday.
"We certainly aren't encouraging that; we believe campus will be safe tomorrow," she said.
Police have interviewed several students who posted the images online and others they believe may have information about the threatening message.
"We know someone out there knows something, and we just want them to come forward," Steward said, adding that the student who wrote the message will not necessarily be punished.
"Our biggest concern, honestly, is the safety of that student," she said. "We want to make sure the student has the services they need and obviously we need to determine how serious the threat might be."
Photos of the graffiti circulating online show a message scrawled in marker that reads, "I f---ing hate everyone at this school!!!! Watch me shoot everyone on March 14th! I swear to God!!!!!!"
Police are continuing to work with the school to investigate the incident, and there will be increased patrols on campus throughout the week, the principal said.
Parents or students with information about the graffiti are asked to call the school as soon as possible at (925) 461-6600 or via an anonymous tip line at (925) 417-5199.