Swastikas and the word "skinhead" were found painted at Kaiser Elementary School's baseball field, officials said.
Principal Deborah Granger said staffers first noticed the slurs when they got to school Monday.
Swastikas were painted on trash cans, and the word "skinhead" with a swastika had been painted on part of the backstop, Granger said. The tops of benches had also been painted white.
By Monday night, the graffiti had been blurred out so it wasn't visible to children or other passersby, and it was fully painted over Tuesday, Granger said.
A usually locked gate to an equipment storage area was left open after a baseball game played at the field over the weekend, Granger said, explaining that other groups use the school's field under a joint use agreement.
Granger said she believes it was a crime of opportunity, with paint grabbed from the open storage area.
"The fact that there happened to be this open ball cage, it just seems like somebody was poking around," she said.
The Newport Mesa Unified School District typically covers graffiti, or tagging, before students arrive at school, unless it's on a mural, in which case the artist is contacted, said district spokeswoman Laura Boss.
The swastikas were painted at a part of Kaiser that wasn't very visible, but in this case, students leaving campus to go to the nearby Boys & Girls Club did have to pass by the vandalism Monday afternoon, the principal said.
"Luckily, we're an elementary school and the students were pretty oblivious to what it meant," Granger said. "It didn't resonate."
It did, however, resonate with adults, she added.
"It's incredibly offensive and it's frightening," Granger said. "And I'm hoping that whoever did it, did it in ignorance of how seriously other people would take it."
Costa Mesa police are investigating but have no suspect or further information at this time, Lt. Greg Scott said.