A Visalia police youth services officer at Golden West High School was alerted to a social media post showing a student with a gun at 9 am.
The student was found and arrested.
Authorities say they searched the student's car in the school parking lot - and found a handgun with no serial number in the center console.
Police say the investigation is ongoing, but there is no threat to students or staff.
They also served a search warrant at the home of the student's father and found an illegal marijuana grow, ammunition, and another gun with no serial number.
The 47-year-old father was arrested on multiple charges.