The San Ramon Valley Unified School District says a student from another school assaulted California High School's assistant principal.
ABC7 News obtained a letter sent by the superintendent to the school community.
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It says the assistant principal approached two students in the California High School's parking lot during Saturday's dance.
One of the students attacked the staff member and ran off.
The district says police arrested a juvenile the same night.
The district believes both students went to another San Ramon Valley school.