Garry (17), the head boy at John McGLashan College, retained the title over the 6km course in a time of 20min 12sec and was followed home by Ben Anderson (Kings) 20min 31sec and Alex Gorrie (Kavanagh) 20min 33sec.
The 18 second-winning margin could have been greater because Garry ran conservatively and stayed with the bunch during the first of the three laps that was completed in 6min 36sec.
Garry, who finished 19th at the world secondary school championships in the Czech Republic last year, knew that the race was his when he broke from Anderson and Gorrie early in the second lap and led by 60m after two laps.
He applied more pressure on the final lap and the gap widened as he sprinted up the muddy front straight.
His next target is the New Zealand secondary schools cross-country at Nelson on June 20.
He finished 15th in the senior boys race two years ago and 10th last year and should be among the medals this year.
The other scoring members were Alex Dodds (sixth), Mitchell Clarke (seventh) and Paddy Beamish (11th).
Sarah McLaren (16) reaped the benefit of the intense competition she gets at St Hildas to win the senior girls title from three of her school mates.
She comes from a sheep farm near Wyndham and has kept fit by running around the farm in her school holidays.
McLaren, who took control midway through the second of the two laps, won by 17sec in a time of 15min 51sec from Louise Harvey 16min 08sec, Amelia Kinnaird 16min 15sec and Laura Mackay 17min 04sec.
The four boarders at Tolcarne won the four-girl teams race for St Hildas.
The girls under-16 event was one of the best races of the day with Anna-Lisa Uttley (Bayfield) applying pressure over the final 800m to win by five seconds from training partner Rebekah Greene (St Hildas) in a time if 10min 54sec.
Katrina Andrews (Otago Girls) was third in 12min 30sec.
Uttley and Greene have both won gold medals at New Zealand secondary school events.
They are both trained by Jim Baird.
His runners filled the first four placings in the girls under-14 race; second, third and fourth in the girls under-15 event, the first three places in the under-16 girls race and the first five places in the senior girls event.
The other winners were: Sam Hollows (South Otago) girls under-14, Anna Kean (Bayfield) girls under-15; Sam Bremer (Kavanagh) boys under-14, Toby Batchelor (John McGlashan) boys under-15 and Cameron McNoe (Kaikorai Valley) boys under-16.
The winners of the other teams events were: Boys under-14, Kings; boys under-15, Kings; boys under-16, John McGlashan; girls under-14, Columba; girls under 15, Columba.
A total of 180 runners competed in the event.
