Despite the threat, morning attendance at the high school was about 92 percent, Pleasanton Unified School District spokeswoman Nicole Steward said.
About 110 students called out of school, and the cheerleading squad and lacrosse team are away at events. Some other students are on school-organized field trips, Steward said.
There are 2,200 students enrolled at the school.
The graffiti, scrawled on the wall of a bathroom stall in marker, read, "I f---ing hate everyone at this school!!!! Watch me shoot everyone on March 14th! I swear to God!!!!!!"
District officials and peer tutors made a point of visiting the high school campus this morning to show the students and parents that the school is safe, Steward said.
Steward said some students are saying they believe the threat was "sensationalized."
Pleasanton police are beefing up patrols at the school as a precaution. Instead of the usual one officer, there are four uniformed officers patrolling the grounds, Steward said.
The patrols are happening around the clock, and the added police presence will extend through Friday, she said.
After the graffiti was found Tuesday, faculty and staff were briefed about the threat at a staff meeting Wednesday, and were instructed to talk to each of their classes about the incident, she said.
All classroom doors were locked once classes started today, but that is normal protocol for the high school, Steward said.
Police have interviewed several students who posted photos of the graffiti online and others they believe may have information about the threatening message.
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.