The Duchess of Cambridge formally opened a new Astroturf during her visit to the school, where she told students she spent some of her "happiest years".
In a speech 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."