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Blustery and bitterly cold conditions in Dunedin on Saturday presented a challenge for both sides.
But McGlashan embraced the challenge and led 24-7 at the break.
Mt Aspiring came back into the contest in the second spell, outscoring McGlashan 12-5.
McGlashan had made some personnel changes but the visitors rallied and performed well.
"We made up our mind that into the wind we would keep the ball in hand," McGlashan coach Matt Direen said.
"We recycled possession really well and were able to go wide and create some holes.
"We got into space really well. But Mt Aspiring came back really strong. They didn't lie down at all."
McGlashan captain Leroy Ferguson played his 50th game for the team last week and celebrated his capping with another strong performance on the side of the scrum, while hooker George Bell was impressive as always.
The win means McGlashan moves into the top four of the Otago premier school standings. It improved its record to four wins from six games.
King's High School climbed a spot as well. It beat St Kevin's College 24-0 to move into fifth place.
Waitaki Boys' High School suffered a 19-12 loss to Dunstan High School and slipped out of the top four.
Otago Boys' High School Second XV beat Wakatipu High School 46-20 and remains third, while the second placed Southland Boys' High School beat the Combined Co-Eds 69-8.
South Otago High School defaulted to competition front-runner Otago Boys' High School First XV. Otago Secondary Schools Rugby Council chairman Greg Heller said South Otago only had 14 players available but it was agreed Otago Boys' would travel down to Balclutha to play a practice match instead.
The game was recorded as a 20-0 win to Otago Boys'.
Earlier this month the scheduled game between Otago Boys' and St Kevin's College was deferred but Heller conceded that game was unlikely to go ahead, due to the busy schedule of interschool games.
Heller said the co-educational schools were "doing their best" to field teams.
"But we know they are a little undermanned. But they are out there and doing their best and I've always said there will be an annual competition review, in terms of looking at what is best for our competition."
The competition is structured so all 12 teams play each other once. The teams then split into a top, middle and bottom four.
The gap between the top sides and the bottom teams has resulted in some lopsided encounters so far.