During her two-hour visit, Kate officially opened the school's new Astroturf and happily reminisced about everything from midnight feasts to the trees she used to climb.
She described her eight years at the school as some of the happiest of her life - and even revealed a hitherto unknown nickname, Squeak.
It appears that St Andrew's had two guinea pigs called Pip and Squeak, and when Pip(pa) and Kate started at St. Andrews, they soon acquired the nicknames for no reason other than names fitted.
Kate was even treated to her favourite school lunch - Scottish roast beef and 'birdseed', a mixture of flapjack and Rice Krispie cake - after staff asked St James's Palace what she would like to eat. Recalling her time at the school, Kate said: 'It is such a treat to be back here at St Andrew's. I absolutely loved my time here; they were some of my happiest years, which makes it so incredibly special to be here today.
'In fact, I enjoyed it so much that when I had to leave, I told my mother that I was going to come back to be a teacher.
'While that didn't quite happen, I was thrilled to have been asked back today on St Andrew's Day.'
- Daily Mail