Dunstan beat John McGlashan College 28-17 in Alexandra to join Otago Boys' High School and Southland Boys' High School in the top four.
John McGlashan College and King's High School are in contention for the fourth spot. The competition has a break for three weeks during the school holidays,before the last two round-robin games are played.
It was a significant performance by Dunstan, which will be the only co-educational school in the top four.
Dunstan and McGlashan were 14-all at halftime, but Dunstan had moved ahead by the middle of the second spell.
Dunstan's tries were scored by centre Canaan Napier, second five-eighth Sam Galletly, fullback Jayden Auburn and loose forward Cody Laing. No 8 Daniel Riley kicked four conversions.
John McGlashan's tries were scored by locks Charlie Beckett and Stan South. Centre Chris O'Connell kicked two conversions and a penalty goal.
Other players to impress were first five-eighth Jacob Sinclair, busy openside flanker Oliver Povoas and strong-running winger Robert Jopp.
A last-minute try by winger Piliu Tavake gave Waitaki Boys' High School a 23-21 win over Menzies College.
It was a move that went through nine phases and covered 60m. Tavake also scored the first try to help Waitaki Boys' to a 15-13 lead at halftime. The other try was scored by lock Don Lolo.
Tighthead prop Liam Sharples will remember his first game for the Otago Boys' First XV after scoring four tries against South Otago High School in Balclutha.
Otago Boys' scored 11 tries - all by the forwards - to win 71-5. It was a good day for the front row, because loosehead prop Jonah Aoina also scored two tries.
Other players to impress were lock Matt Heffernan, No 8 Sekonaia Pole and fullback Matt Jones.
In the bottom section, St Kevin's College beat Mt Aspiring College 53-10, St Peter's College beat Verdon College 15-10, and Gore High School beat James Hargest College 20-16.
