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."
More top news
Winds will veer northerly overnight with colder air streaming in over the next few days.
Protesters angry that far-right leader Marine Le Pen is advancing the French presidential runoff are scuffling with police in Paris.
Polling projections show the far-right leader Marine Le Pen and centrist Emmanuel Macron leading in the first-round of the French election.