According to an update issued by the school district shortly after 3 p.m., there was not an active shooter at the school and no injuries were reported.
Police officially declared the campus safe around 4:30 p.m.
Officers first responded to the school at 520 Walnut Street shortly after 1 p.m. and a bomb squad was summoned to the scene, according to police.
A suspect was initially believed to be a student, a police spokesperson told ABC7, but investigators later said a suspect was not found.
News video from AIR7 HD showed officers who arrived at the campus in several patrol and SWAT vehicles.
Students have since been released.