McGlashan led 41-6 at halftime and ended the game with 13 tries. It was the most tries the team has scored since the Highlanders First XV competition started five years ago.
Centre Seveci Nakailagi Naisilisili was devastating with the ball in hand and scored three tries.
No 8 Nemo Gent scored two.
McGlashan dominated all phases of the game and set the tone early, scoring four tries to lead 24-0 after just eight minutes.
Other players to impress were openside flanker Rory Ferguson, lock Woody Kirkwood, second five-eighth Josh Timu and wing Josh Preston.
Midfield back Sio Tomkinson was the star of the Otago Boys' High School team that thrashed South Otago High School 92-7 in Balclutha.
Tomkinson kicked 11 conversions from 12 attempts. Otago Boys' scored 13 tries, with prop Jonah Aoina and centre Hemaua Samasoni each scoring two.
The best try was a brilliant 60m solo effort scored by blindside flanker Matt Heffernan.
King's High School scored eight tries to beat Mount Aspiring College 52-14 in Dunedin.
Second five-eighth Reece Te Pairi had a sparkling game on attack and his strong tackling blunted Mount Aspiring attacks. His goal kicking was also on target and he landed six conversions.
The quick delivery of halfback Joel Meade gave his outsides space and he also made some telling breaks.
Brodie Flannery kicked two penalty goals as St Kevin's College ground out a narrow 6-0 win over Dunstan High School in Oamaru.
Dunstan dominated the lineout and had more possession, and it was only the strong St Kevin's defence that won the game.
Other players to impress for St Kevin's were halfback Scott Weir and wing Connor Newlands.
In other games, Southland Boys' High School beat Gore High School 67-7, Waitaki Boys' High School beat St Peter's College 46-7, and Menzies College beat James Hargest High School 20-3.
