Prince William's wife was invited back to the facility, in Pangbourne, Berkshire, to celebrate St Andrew's Day with pupils and staff. She opened a new hockey pitch - and tested out the surface herself - before giving a speech in which she described the nine years she spent there as the "happiest years of my life".
She 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.
"It was while I was here at school that I realised my love of sport. Sport has been a huge part of my life, and I feel incredibly grateful for the opportunities I had to get outside and play in such wonderful open spaces - though sadly there was nothing quite as glamorous as this in my time!
"I hope that you all enjoy playing sport here as much as I used to, and make the most of these incredible facilities."
It's not the first time the royal has shown of her skills with a stick. Back in March she ditched her heels for a knockabout with the British hockey team, ahead of the Olympics. And she was also spotted sharing the highs and lows of Team GB's hockey players as they competed at London 2012.
Among the staff present was Deputy Head teacher Richard Hudson, who remembered Kate from her time at the school. He described her as a "sporty" pupil, telling the BBC: "She held our record for the high jump... for about 20 years she's had that. She was very hard working, very polite and friendly, [and] very kind to the younger children.
As well as sports, the 30-year-old was also keenly involved in other aspects of school life, including public speaking competitions and drama groups. In the run up to the royal wedding last year, much excitement resulted over a video which surfaced of Kate taking part in a school play during her time at St Andrew's. In the clip, her character is told by a fortune teller she will marry a prince and he will fall in love with her. This of course, came true.
Fittingly, to celebrate the day of the patron saint of Scotland, the elegant brunette wore a tartan ensemble, made up of Black Watch tartan, which is one of the most popular designs of the Scottish fabric.