Earlier, centre Matt Whaanga had got South Otago back in the match with a sensational solo 50m run to score in the right corner, after McGlashan led 20-12 with about 15 minutes to play at the Balclutha Showgrounds.
In a game which featured several shifts of momentum, Whaanga broke the McGlashan defence near halfway and outpaced the cover defence to score.
Whaanga, midfielder Hemi Remi and fullback Rory van Vugt were outstanding for the home side, capitalising on good performances by a hard working forward pack that regularly stole turnover ball.
McGlashan was well served by blindside flanker Rory Ferguson and second five Josh Timu, while locks Woody Kirkwood and Sam Moir-Grace toiled hard.
South Otago led 12-10 at halftime, after John McGlashan took an early 7-0 lead.
In other matches, the Otago Boys' High School Second XV beat King's High School 29-26 in a tense match at Littlebourne, Dunstan High School beat Mount Aspiring College 40-33, Waitaki Boys' High School beat Taieri College 29-25, and Kavanagh College beat St Kevin's College 21-20.
