She is pictured with St John national director of ceremonies Ian Rae, of Wanaka, and the rest of the youth division after the presentation.
The Grand Prior badge is on the left upper arm of her uniform.
Mr Rae said the award represented a lot of work and was a significant achievement.
"Grace has shown perseverance and huge tenacity in getting there."
Grace, a year-12 pupil at Dunstan High School, said her plan after completing her secondary schooling was to study for a bachelor of health sciences degree at the Auckland University of Technology, majoring in paramedicine.
"Then I want to go back to working at St John’s."
Alexandra divisional manager Darryn Drake said Grace was the first cadet in three or four years in Alexandra to gain the Grand Prior award.