Long Beach police said once they were notified of the post, they investigated and arrested a juvenile they believe is responsible for the post.
"At this time, the department is confident there is NO threat to the high school," the department said in a statement.
The exact nature of the threat was not immediately known.
Police said there will be extra patrols visible around campus on Tuesday out of an abundance of caution.
The identity of the student arrested was not disclosed.