According to Express.co.uk, Kate managed to attend the low-key event without being seen, by arriving and departing via a back entrance to the establishment.
A doorway in a back alley meant that Kate could get in and out without using the main entrance, and avoid any photographers.
This is Charlotte's first term at the school, which her brother George first joined in September 2017.
Both Kate and Prince William dropped their kids off for their first day in September, with the children looking smart in their navy blue uniforms.
When it was announced that Princess Charlotte would also attend the school, Simon O’Malley, headmaster at Thomas’s said: “We are delighted that the Duke and Duchess of Cambridge have decided that Princess Charlotte will join her elder brother, Prince George, at Thomas’s Battersea.
“We greatly look forward to welcoming her and all of our new pupils to the school in September”.
The Duke and Duchess have recently returned from a royal tour to Pakistan.
Their visit, between October 14-18 largely focused on showcasing Pakistan as it is today – a dynamic, aspirational and forward-looking nation.
They visited Islamabad, Lahore, and the mountainous countryside in the North during their trip.