The key players in the King's team were loose forward Ben Dolan, prop Ayden Lloyd and utility back David van der Byl.
Kavanagh scored first when first five-eighth Ben Willemsen broke the line.
King's tightened its defence after this and the forwards were a tight unit and provided good ball for an enterprising backline that scored four tries to lead 29-7 at the break.
Openside flanker Tyrone Pelasio had a lively game and scored two first-half tries.
Tighthead prop Jack O'Kane and midfield back Joe Muasika impressed in the Kavanagh team.
John McGlashan College finished 10th last year but started the season in style by beating South Otago High School, last year's Cinderella side, 37-28.
McGlashan scored seven tries with No 8 Nathan Hull scoring two.
He is a member of the New Zealand Under-17 squad.
Loose forwards Nick Ryder and Christian Schaaf also impressed.
The best South Otago players were hooker Karl Still and tighthead prop Shane Edwards.
Otago Boys' High School went on the rampage to score 16 tries when it beat Central Southland College 92-0.
Fullback Liam Edwards scored four tries and No 8 Te Reo Apiapo two with his bullocking runs.
Southland Boys' High School had the other blow-out score when it beat St Kevin's College 81-0.
Waitaki Boys' High School beat Dunstan High School 17-12, and Gore High School beat Verdon College 19-6.
