Twitter followers commented on Princess Charlotte's cuteness and her royal bearing.
The Queen was homeschooled at Buckingham Palace by private tutors and teachers, so she doesn't have first day portraits like her children, grandchildren, or great-grandchildren do.
The tweet was followed by confirmation that the pictures had been taken by Kate Middleton as she sent her daughter off to school for the first time.
She had a little backpack on her back, and was beaming with excitement, showing none of the nerves that her older brother, Princes George, 4, displayed when he arrived for his first day of primary school past year.
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The school, which charges £14,550 ($19,740) per year, was classed as "outstanding" at the last official inspection in 2012.
It has an unflinching ethos according to its website: "High standards, excellence and good manners".
Charlotte will be full-time at Willcocks, at a cost of more than $AUD5000 per week. A special "lunch club" offers French lessons and "local outings". Visitors include emergency services workers, and Prince William will be able to swap stories with other dads at "fathers' breakfasts".
A spokesperson for the school said at the time, "We are delighted that The Duke and Duchess of Cambridge have chosen the Willcocks Nursery School for Princess Charlotte".
"We look forward to welcoming Charlotte to our nursery".