Some tenacious defence from Southland Boys’ and a strong effort in lineout were also major factors.
The visitors had to survive a strong finish from McGlashan but held on to win 17-15 in Dunedin.
They took advantage of early lineout problems to dominate possession. But it was their scrum which set up the first try.
Southland Boys’ pushed the McGlashan pack off its ball and got the pill wide to dangerous winger Richie Kuresa to score.
Classy first five-eighth Kaea Nikora-Balloch drilled the conversion and was in the action again five minutes later. He jigged his way through to score and slotted the extras.
The home side eventually found some rhythm. It grabbed its lineout ball which helped and took advantage of some field position to score close to the corner.
The conversion hit the post which proved costly in the end.
And moments before halftime, lock Harrison Morton got as close as you get to scoring without actually scoring. He stretched out one long limb for the try line but grounded the ball on a stray boot. The opportunity was lost.
Southland stretched its 14-5 lead early in the second spell with a penalty.
After that it was pretty much all the home team. Centre Elias O’Kane made a strong run down the left flank and tighthead prop Liam Ferguson crashed over to narrow the gap.
The home team battered away at the line. Eventually replacement forward James Paterson got over but the conversion sailed past the near post.
In the other matches, Otago Boys’ beat Mt Aspiring College 28-0, scoring all its points in the first half, while King's dispatched Dunstan 73-7.
In the championship competition, Otago Boys' Second XV edged South Otago 17-15, St Kevin's beat Wakatipu 22-7 and Waitaki Boys' beat Otago Combined co-ed schools 43-7.