It beat South Otago High School 19-3 in a tough Otago premier schools encounter at the Balclutha Showgrounds on Saturday.
South Otago needed to win to make the semifinals but could not stick with the visitors in the second half.
McGlashan captain Rory Ferguson scored two tries in his 50th game for the First XV to help clinch the victory.
The school had already sealed top spot and the competitive match was just what the team needed heading into this weekend's semifinal match.
It will host Otago Boys' High School Second XV after it slipped down the competition table one place following a 41-31 loss to Waitaki Boys' High School in Oamaru.
Waitaki had already secured second place and will host King's High School in the other semifinal. King's beat Dunstan High School 55-12 to finish the round-robin in third place.
In the bottom six, Kavanagh College beat Otago Boys' Third XV 59-7, Mount Aspiring College beat South Otago Second XV 39-10 and St Kevin's College beat Taieri College 53-10.
