The black and white snap sees the screen legend, who has appeared in more than 55 feature films, sitting among her girl classmates at Hereford House in Ilfracome.
"Me at school in the dark ages #throwbackthursday - anyone recognise anyone else in this picture?, she captioned the photo.
Unfortunately Joan didn't point out to her legions of fans where she's situated in the snap.
Collins will be playing Queen Helena's (Liz Hurley) mum the Grand Duchess of Oxford in the fictional monarchy drama.
The reality scripted drama will see the fake monarchs trying to attempt to master the art of living the royal life in modern-day London where social media is just a click away.
In a promo shoot for the show Joan is seen looking regal in purple dress as she sits alongs her daughter Queen Helena and her rebellious granddaughter Princess Eleanor.
She said: "You need to want to see drama and heightened stuff and a lot of glamour. If that's your bag, you might get lucky."
The Royals is set to premiere in 2015.