Like all royals, Prince George has age-appropriate responsibilities to fulfill; in fact, it was an important duty that prevented the young royal from attending the wedding of his godfather, the Duke of Westminster, who recently married Olivia Henson.
Since the wedding of the Duke of Westminster took place on a Friday, it was impossible for the future Prince of Wales to be present since it was an ordinary weekday, in which the eldest son of Prince William had to attend school as usual.
While it is true that Prince George has been seen more involved in the public events of royalty accompanying his father, his only duty now is to successfully complete the school year because, due to his young age, he has no other responsibility regarding his title of prince.
It is for that reason that Kate and William took the decision that their firstborn son would not attend the wedding of his godfather, the Duke of Westminster, since for them the priority is that their son attends school and stays responsible with his academic activities.