Princess Charlotte is growing up so fast.

The 4-year-old daughter of Prince William and Kate Middleton started school today, waving and smiling at photographers as she walked beside her big brother Prince George, 6, through the gates of St Thomas's in Battersea in south London.

Just two years ago, it was a shy George arriving at the school gate with his father โ€” his mother missed the occasion as she was experiencing severe morning sickness in her pregnancy with Prince Louis.

Princess Charlotte arrives at school with her brother Prince George. Photo / AP
Princess Charlotte arrives at school with her brother Prince George. Photo / AP

Luckily for Charlotte, Kate was around to join William on the school run โ€” whose hand she clutched nervously as she approached the classroom, fiddling with her ponytail as she walked.

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The Kensington Palace Instagram account also posted the sweet first-day photos, and a video of her meeting one of her teachers to mark the momentous occasion.

St Thomas's in Battersea has 560 students aged from four to 13 and the ethos "be kind".

Pupils can learn everything from art to ballet, drama, French and music with specialist teachers.

Helen Haslem, head of the lower school greets Britain's Princess Charlotte as she arrives for her first day of school at Thomas's Battersea in London, with her brother Prince George. Photo / AP
Helen Haslem, head of the lower school greets Britain's Princess Charlotte as she arrives for her first day of school at Thomas's Battersea in London, with her brother Prince George. Photo / AP

Headmaster Simon O'Malley said the school emphasised key values such as "kindness, courtesy, confidence, humility and learning to be givers, not takers".

"We hope that our pupils will leave this school with a strong sense of social responsibility, set on a path to become net contributors to society and to flourish as conscientious and caring citizens of the world," he said.