The shooting happened Wednesday evening in a parking lot at a campus shared by Sem Yeto and Fairfield High School. Authorities said roughly 1,000 people were attending the graduation ceremony at Schaefer Stadium when gunfire broke out around 7:15 p.m.
An 18-year-old was killed, and three others - ages 11, 20 and 25 - were also struck. Police have not identified the victims and said no arrests have been made.
Students returned to campus Thursday to complete final exams, describing the experience as difficult in the aftermath of the violence.
"If I am being honest, not really. Like a little, but not really," one student said when asked about feeling safe. The student added, "I did see some police officers, but like, that was it."
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She added, "I didn't want to come, but I had tests to finish and stuff, because they didn't excuse anything."
"It's really tragic. It's like we still have to go (to school)," another student said. "I thought they would at least like cancel school or something, but we still have to go."
Students said they were familiar with the victim, who had just graduated from Sem Yeto Continuation High School.
A nearby resident described hearing the gunfire and the immediate reaction.
"That's what I heard, was the pops and then the screaming, 'Oh no!' And then I came out and I saw all the people rushing away from where the actual incident happened," said Shawn Richardson, who lives across the street from the school.
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The Fairfield Police Department declined an interview request but said in a statement that investigators are actively working the case.
"We understand this incident has caused significant concern within our community. While the investigation remains ongoing, Fairfield Police investigators are working diligently to identify those responsible and ensure the continued safety of our community," the department said.
The Fairfield-Suisun Unified School District said Friday night's Fairfield High School graduation has been moved to another school "out of respect for the community." The district also emphasized that officials do not believe there is an ongoing threat.
"We recognize that incidents like this can create feelings of fear, uncertainty, sadness, and anger. We wanted to reiterate what the Fairfield Police Department has shared with us: that there is no indication of an ongoing threat to our schools or the community," the district said in a statement.
Authorities are asking anyone who attended the graduation and may have information to contact police as the investigation continues.