Kraemer Middle School's Assistant Principal Moises Plascencia apparently shot himself in a private staff area, and no one else was present at the time, police confirmed.
The Placentia Police Department was called to campus after he was found.
"Words will never be able to express our true sorrow regarding this loss," Principal Michael Young wrote in a message to the school community. "While we may never make sense of nor understand why this occurred, please know how deeply Mr. Plascencia cared for your students, their education, and all of Kraemer Middle School."
School operations were suspended for the rest of the day, but are expected to resume Tuesday on the regular schedule.
The Placentia-Yorba Linda Unified School District is making mental-health professionals available on campus to provide support for students, staff and families.
If you or a loved one have contemplated suicide, call the National Suicide Prevention Lifeline at 1-800-273-8255 or text TALK to 741741. The International Association for Suicide Prevention and Befrienders Worldwide also provide contact information for crisis centers around the world.