Police said the parent, reported to have had a breakdown, barricaded himself in a classroom after some sort of confrontation with a teacher.
The teacher who was barricaded with the man is believed to be 70-year-old Linda Montgomery, according to KABC.
The Riverside Unified School District posted this message on their social media accounts and website:
"At this time Castle View Elementary has been placed on lockdown due to a disturbance inside of a classroom. Riverside Police Department is on site and actively working to resolve the situation. All students are being evacuated to a safe location off-campus. Unfortunately, at this time we cannot release any students until the lockdown order has been lifted by RPD. We will let you know as soon as the situation is resolved and give further instructions on student pickup. Thank you for your cooperation."
A parent is barricaded in classroom with another adult. Police say all students are evacuating and safe. pic.twitter.com/VeX6GVetz9