Charlotte, who's now four years of age, is now attending Thomas's Battersea in London, and she posed alongside her older brother, Prince George, just outside their home, Kensington Palace.
The palace released an official statement along with the photos of Princess Charlotte saying: "The Duke and Duchess of Cambridge are very pleased to share a photograph of Prince George and Princess Charlotte at Kensington Palace this morning. The photo was taken shortly before Their Royal Highnesses left for Thomas's Battersea."
Charlotte was dressed in her brand new navy school uniform, which is the school's traditional uniform.
The uniform consists of a blue pinafore with a white summer blouse, and it's topped off with black shoes and white socks.
George, who's now six years old, was also dressed in his navy school uniform.
In the rather sweet photo, the two siblings had their arms around one another and were smiling up at the camera.
Prince George is now in Year 2 and it was a very special day for the Duchess of Cambridge as she wasn't able to attend her son's first day at school back in 2017 since she had a severe case of morning sickness while she was pregnant with her second son, Prince Louis.
However, it was all smiles today and the two children walked hand in hand with their parents to their first day of the new term after the summer holidays.
It was revealed that the royals were greeted by the head of the lower school, Helen Haslem, as they entered through a side entrance as a precaution.
There was a lovely moment as the school's headteacher bent down to shake hands with both George and Charlotte.
Kate looked very proud of her daughter as she held her hand and couldn't stop herself from beaming.
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