Michael Jackson's Son Prince -- Grandma & Grandpa Reunite for Graduation Party (PHOTOS)

Joe and Katherine Jackson don't get together often, but they put aside their differences to celebrate Michael Jackson's firstborn graduating from high school.

According to our sources, the family shindig for Prince Jackson went down Saturday afternoon at Joe's house in Encino. We're told it was a small gathering -- about 35 people or so -- with Prince's aunts, uncles and cousins making up most of the crowd.

The party was catered, but just typical party food ... nothing too fancy ... especially considering how filthy rich the guest of honor is. Uncle Jackie and Grandpa Joe made speeches, along with Prince.

Prince graduated Saturday from the exclusive Buckley School -- and says he's going to LMU School of Film and Television ... following in the footsteps of other celeb kids Colin Hanks and Scott Eastwood.