Former New Zealand Secondary Schools rep Stephenson was the player of the match.
"Her composure and leadership ... she had a blinder," Kupa said.
NGHS led by four goals going into the final quarter. However, when Kupa switched Stephenson from centre to wing attack and her Year 10 daughter, Parris Petera, from wing attack to centre, NGHS were able to increase the tempo and HGHS were unable to match it.
Kupa was able to play all five of her subs during the final five minutes. Her team finished with a 79 per cent success rate at goal while Hastings could only convert 56 per cent of their opportunities.
The Napier shooters sank 35 of 44 attempts while the Hastings shooters converted 24 from 43. Napier's starting goal shoot, Janaya Preece, had a 77 per cent success rate and starting goal attack Janayah Lewis had an 82 per cent success rate. Baker succeeded with 80 per cent of her attempts.
Hastings led 9-8 after the first quarter, Napier led 17-13 at halftime and 22-18 going into the final quarter. Kupa hopes to be able to coach NGHS again next year.
"I'm due to have another baby on Christmas Day which means I will have two young ones under 2 so I will have to see if I can fit everything in," she added.
"We were right there until the final quarter in what was an awesome game," Hastings Girls' High School coach Ina Jones said.
"But during that final period they put us under tremendous pressure and forced us into errors. They were amazing."
Captain and goal defence Liana Mikaele-Tu'u was Hastings' player of the match. Hawke's Bay under-17 rep Summer Rose Taylor was a tireless worker in her favourite position of wing defence.
"We had the same number of scoring opportunities as Napier but didn't convert them. That was a tribute to the defensive pressure the likes of Jaydi [Taylor-Chaffey] put us under," Jones added.
Hastings will be 12th seeds at the Lower North Island tournament in New Plymouth.
Taradale High School pipped Sacred Heart College 32-28 in the plate final and Wairoa College beat Karamu High School 24-22 in the bowl final.