Openers Cohen Loffler (65) and Briant justified that call with a 113-run stand in 22.2 overs — the highest Boys' High partnership for any wicket to date this season.
Loffler's 42-run stand with first-drop Nathan Trowell, broken by Priyanka Vithanage (1-16 off three overs) with the first ball of the penultimate over, was another strong showing.
Trowell (27 off 26 balls) and No.4 Travis O'Rourke (one off two) closed the innings at 167-2, the students highest total on either side of Christmas.
Seamers Vithanage and Kavinda Mudiyanselage (1-17 off three), both skilful bowlers, were Horouta's wicket-takers.
The biggest moment of Horouta's chase came when GBHS leg-spinner Dylan Foster (1-31 off four) got the ball to zip from the wicket and Ranasinghe (4 runs) chopped the ball on to his stumps off the second ball he faced.
Horouta were 87-3 in the 20th over and the game was up for grabs.
Off-spinner Stewart (1-21 off 5) and opening bowler Matthew Foster (1-30 off 6) were also steady, and the GBHS effort in the field was a major factor.
Luke Fisher's speed to the ball out deep and effort to return the ball quickly was outstanding.
Horouta first-drop Mel Knight's 51 from 56 balls was a thinking player's knock brought to an end by off-side sweeper Daniel Watts' accurate return to keeper Loffler to run her out with the score at 142-7 in the 28th over.
Hasantha Vithanage became the fourth run-out victim — with the score at 157-9 — off the last ball of the match.
“Our openers were still in at drinks, got 50s and then we bowled well, not giving Horouta many scoring opportunities,” said Stewart.
“Left-arm medium-pacer Lukas Fry (0-13 off four) especially kept things tight. Everyone contributed in some way, which was great to see.”
Ranasinghe praised the hard deck on which Briant concentrated successfully on bowling outside off-stump.
“There was some movement but we lost the game because of dot-balls,” said the Horouta captain.
“GBHS ran quick singles and rotated strike in the first innings. That's what we didn't do.”