And after photos taken by the Duchess of Cambridge were released by Kensington Palace to mark the special occasion, all eyes were on the little girl's adorable outfit - especially her pink pony print rucksack.
The £14,500-a-year school is owned and run by headmistress Lavinia Taylor.
The choice for Charlotte is in contrast to the nursery picked for brother Prince George, who was sent to Westacre Montessori School near the Cambridges' Norfolk home, Anmer Hall.
According to its website, Willcocks is a "traditional nursery school which strives to maintain its ethos for high standards, excellence and good manners".
The afternoon school has fees of £1,800 per term and is less structured than the morning session but provides a similar wide range of activities including art and music.
It said: "The organisation of the time that children attend is exceptional".More news: Bannon: Treason comment in new book not about Trump Jr
More news: Rams schooled by Falcons in return to playoffs
More news: Mariota leads Titans to 22-21 comeback win over Chiefs
Tatler called the nursery a "hidden gem" that "flies under the radar".
A spokesperson for Kensington Palace added: 'They felt it was ideal as a first step for Princess Charlotte's early education and they were impressed by the team that work there'.
He is now a pupil at Thomas's Battersea and started there in September 2017.
The school was founded in 1964 and has been at its current location near the Royal Albert Hall since 1984.
The Duke and Duchess of Cambridge are very pleased to share two photographs of Princess Charlotte at Kensington Palace this morning.
It feels like it was only yesterday that we were welcoming little Charlotte into the world.