Brandon Evans, who is the son of the current Mrs. Tennessee, Melissa Evans, also faces being expelled from high school just weeks away from graduation.
School leaders say sheriff's deputies got a tip about Evans having drugs at Powell High School in Knoxville on Thursday.
They didn't find drugs when they searched his car, but did discover a 45-caliber handgun under the seat and arrested Evans.
School spokesman Russ Oaks says there is a zero tolerance policy for guns, so Evans can't come back to school.
Brandon Evans' father John Evans told WBIR-TV in Knoxville that the gun was his and he put it in the car after a weekend outing. He says his son did not know it was there.
John Evans said he and his wife plan to fight their son's possible expulsion.