Prince George will start at Eton College in September, a path set in motion decades ago by the grandmother he never met, Princess Diana.

George, 13, is following his dad Prince William‘s footsteps in attending Eton, but it was an unexpected choice for William to attend the all-boys boarding school located not far from Windsor Castle. Previous male members of the British royal family — including William’s father, King Charles, and grandfather, Prince Philip — attended Gordonstoun in the Scottish Highlands, known for its strict discipline and rigorous demands of students.

Author Simon Vigar of the new biography The Four Wives of Windsor tells PEOPLE that Prince William going to Eton was a break from royal tradition — and “very much a Diana decision.”

“Her brother and father went there, and of course, it was much closer to home than Gordonstoun, where Charles wasn’t happy anyway,” Vigar says.

Despite Charles and Diana’s separation in 1992, both parents were present when Prince William started at Eton College in September 1995. Prince Harry, who would later join his elder brother at the school, also attended the first day of school festivities, including signing the traditional Entrance Book.

Prince William had a “good time” at Eton, says Vigar, and it’s also where he found refuge after Diana died in 1997, when William was only 15.

“Eton was a much-needed haven for William,” royal biographer Sally Bedell Smith recently told PEOPLE.

Thirty years later, Prince William and Kate Middleton‘s eldest son will soon be starting at Eton College. Vigar says Eton “ticked so many boxes for William and Catherine,” who explored many options for George’s schooling.

A source close to the family recently told PEOPLE, “George has always wanted to follow in his dad’s footsteps.”

Founded in 1440 by King Henry VI, Eton College has educated generations of leaders, including 20 British prime ministers. In September, George will become the latest royal to don Eton’s famous uniform, consisting of a black tailcoat, waistcoat and pin-striped trousers.

Bedell Smith said, “Yes, Eton stands for tradition, but it has moved with the times and now has a more diverse student body than it had — even in William’s day.”

She added that she believes the Prince and Princess of Wales ultimately made their choice because of Eton’s “academic excellence and the many ways it develops its students for leadership roles.”