Video report by ITV News Royal Editor Chris Ship
Princess Charlotte arrived for her first day at school accompanied by her parents, the Duke and Duchess of Cambridge, and older brother, Prince George.
Charlotte who celebrated her fourth birthday on May 2 received a handshake from the headteacher.
Prince William said his daughter is "very excited" about her first day of school.
The young princess follows in the footsteps of her big brother, who has been attending Thomas’s Battersea for the past two years.
Charlotte appeared uncharacteristically apprehensive as she begins her school career, holding on to her mum's hand tightly as they walked up to greet the headteacher, Helen Haslem.
The duke held on to Prince George's hand as the family approached Ms Haslem.
The duchess smiled and asked: "How are you? Did you have a nice summer?", and Ms Haslem bent down to shake George's hand and then Charlotte's.
William beamed and said: "First day - she's very excited."
The school in south London teaches pupils that kindness is as important as academic success.
Thomas’s Battersea, which describes itself as a Christian school, open to children of all faiths, believes in “praise as the greatest motivator”.
George, who is six, started at the school in September 2017, where he was accompanied by dad, Prince William.
Kate missed George's first day as she was suffering from severe morning sickness while pregnant with Prince Louis.
The young prince looked apprehensive at the prospect of starting at school, but two years on and he is joined by his younger sister.
Pupils are taught art, ballet, drama, ICT, French, music and physical education by specialist teachers.
The school is housed in a Grade II listed building, the former Sir Walter St John’s Grammar School which dates back to 1700.