The 16-year-old arrived at Chill Factore, in Manchester, and put down a great first run of 14.04 seconds.
No other senior females were able to exceed this so Grace retained her title as the British champion for the second year running.
Grace said: “It was a particularly momentous result as it's my last year representing Central Lancaster High School.
"I'm happy to leave this as my high school legacy.
"Central has been amazingly supportive of my skiing and have always celebrated my results."
The British Schools indoor Championship is run by the National Schools Snowsport Association and sponsored by the Ski Club of Great Britain.
Grace is sponsored by Chill Factore, Ski TV official and Anything Technical and trains with Kandahar Ski Racing and GB Ski Cross Squad.