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It edged last year's beaten finalist John McGlashan College 26-13 in a tense encounter in Invercargill.
The scores were locked 13-13 with about 25 minutes remaining.
Southland Boys' co-coach Peter Skelt made a few changes and it helped.
''The bench gave us some impetus,'' he said.
''And it started to rain a little bit. Field position became quite important and I think our decision-making around playing at the right part of the field worked out well.''
Southland Boys scored a try with about 20 minutes to go and the visitors were not able to respond.
Southland Boys' openside flanker Jack Wilcox had a busy game. He tackled relentlessly and was seemingly always over the ball, while halfback Jack Murrell distributed well and made some good decisions.
In Oamaru, King's High School beat Waitaki Boy's High School 26-7 in another keenly contested fixture.
The score was perhaps closer than many would have anticipated but King's coach Ryan Bambry was satisfied his side had picked up a bonus point win.
''We couldn't get any flow on and struggled with some of the interpretations,'' Bambry said.
''We tried to fix that at halftime and got better there. But there is plenty to work on.
''But Waitaki played pretty well. They're a physical bunch and they got stuck into us.''
In the other game, Mt Aspiring College scored an unconverted try in the final moments to beat Wakatipu High School 24-22 in Queenstown.
There was a lot of action in the final 10 minutes. Wakatipu led 15-14 and, despite losing a man to the bin, scored to stretch the lead to 22-14.
But the visitors scored twice to snatch a dramatic win.