In the other match, Otago Boys' High School beat King's 23-13 before a crowd of 2000 in an exciting exhibition of schoolboy rugby at Carisbrook.
The star of the Kavanagh team was dynamic winger Joseph Muasika, who scored two tries. The last try was the best and started on the Kavanagh 22m. Muasika sped down the touchline, brushed off the tackles and scored under the posts.
John McGlashan led 12-10 at the break and looked to have the game in hand with a 31-17 lead. The teams share the Tom Casey Memorial Challenge Cup.
Other key players for Kavanagh were first five-eighth Ben Willemsen, flanker Michael Edh and hooker Joe Weatherall.
No 8 Nathan Hull scored two tries and was the best of the McGlashan players. He was backed by flanker Nick Ryder and winger Sam Sinclair.
A depleted King's team gave a gutsy display to lead 8-5 at the break. It took the lead after 33 minutes when powerful No 8 Tyron Pelasio burst over the line after a 10m drive.
King's played controlled rugby in the first spell, with prop Ayden Lloyd powerful in the front row, Caleb Gray an efficient halfback and fullback Gifford Henry dangerous on the counter-attack.
A rejuvenated Otago Boys' pack moved into top gear in the second spell and attacked as a unit.
Prop Isaac Gutschlag was strong on the drive and flankers Brodie Hume and Josh Reid aggressive in the loose.
Lock Simon Chittock made five clean lineout takes and midfield back Michael Collins scored a brilliant solo try. Fullback Liam Edwards had speed on the counter-attack.
Two penalties to first five-eighth Lance Taylor and tries to wing William Scorgie and centre Makisua Tanuvasa stretched the Otago Boys' lead to 23-8.
Sustained pressure led to a late try for King's replacement Jade Wilson.
King's loose forward Junior Faavae failed in his appeal to the Metropolitan judicial committee to have his two-match suspension overturned.
All eyes will be on Oamaru this afternoon for the traditional First XV clash between Waitaki Boys' High School and St Kevin's College.
FIRST XV RUGBY - THE SCORES
• Otago Boys' High School 23 (William Scorgie, Michael Collins, Makisua Tanuvasa tries; Lance Taylor 2 pen, con), King's High School 13 (Tyron Pelasio, Jade Wilson tries; Gifford Henry pen). Halftime: King's 8-5.
• John McGlashan College 31 (Nathan Hull 2, Braydon Pennycook, Nick Ryder, Tom Scanlan tries; Alan Steel 3 con), Kavanagh College 31 (Joseph Muasika 2, Guy Woodhouse 2, James Tomkinson tries; Ben Willemsen 3 con). Halftime: McGlashan 12-10.