Detectives launched the investigation after reports of the post, which showed an apparent weapon, were received shortly after 11 p.m. Monday, Costa Mesa police said in a statement.
One of the suspects was identified as Lenny Vega. Vega is not an Estancia High School student. The girl was identified as a 17-year-old Estancia High School student. Both Vega and the student were arrested for terrorist threats.
Investigators determined there has been two posts. One showed a subject holding what appeared to be a rifle with the caption "Don't go to school tomorrow." Another post indicated there would be a mass shooting at the school.
Police searched two homes and said they found and confiscated a BB gun pictured in the post.
"In light of these arrests, there is no credible threat to Estancia High School," police said on Twitter.
.@CostaMesaPD says two people are under arrest following a threat made last night against #EstanciaHighSchool on social media. There will be increased police patrols today on campus. @ABC7 https://t.co/mhGZ6mTOCo— Jessica De Nova (@abc7jessica) December 3, 2019
Estancia's resource officer, administrators and Newport-Mesa Unified School District were made aware of the potential threat, according to a news release.
One parent chose not to send her child to school Tuesday and advised other parents to be in constant communication with their kids.
"I'm very social with my kids. I'm very open. I talk to them about everything and I think people don't talk to their kids about certain things because they think it's shameful. I just say 'talk to your kids about everything'," said Araceli Franco.
The matter led to an increased police presence at the campus and more patrols in the area, officials said, adding that the school district had notified staff, parents and students.