"They are very keen to allow George to spread his wings as a child and won't ever do what people expect them to do when it comes to their children."
George and his sister Charlotte, 6, currently go to Thomas's Battersea in southwest London.
Both public school Marlborough and private institution Eton take on pupils from the age of 13, so Catherine and William - who also have 3-year-old Prince Louis - have a few years to decide just yet.
Third-in-line to the British throne, George started school in 2017, and settled in "really easy".
Speaking about his son's first few days at school to patients at Aintree University Hospital in Liverpool, The Duke of Cambridge said at the time: "It's been an interesting week.
"George has been really easy. He hasn't said, 'Have I got to do this for
the rest of my life?'"