I completely feel you!!
My son is in 8th grade. He is super smart. His sister tested into ELP (gifted) and I'm certain he would too, if he had any interest. He is perfectly content to just get by, staying under the radar as much as possible. He doesn't care much about grades. Us saying we will pay him for his grades has helped somewhat. He went from C's and D's to A's and B's but I know he could be a straight A student super easy.
The other day I got a call from a teacher. Instead of turning in an outline for a paper my son turned in 5 pages of dialogue from the movie Shrek.
He swears it was a mistake and he copied it into his paper instead of a new Google Doc, but I don't know if I believe that.