Police took 24-year-old Angel Rojas Jr. into custody after a recording device was found at Martin Elementary School. It's not known if the device recorded anyone in the restroom.
The camera was sent to an FBI crime lab for forensic analysis.
Rojas has worked as a substitute custodian for the Santa Ana Unified School District since June.
"The safety of students is always our first priority of the District, so I commend our School Police Department for responding so quickly to this matter. I also want to ensure our school community at Martin Elementary that this incident is a top priority and as soon as we get more information on the investigation, we will update them," said Superintendent Thelma Melendez De Santa Ana.