Grammar jumped out to an early 8-0 lead but Tauranga came back with a deserved try to combative halfback Leroy Carter and fine conversion from Cole Forbes to trail 8-7 at the break.
A breakout attack from Tauranga deserved a try but outstanding ball runner Sione Faiva had the greasy ball dislodged from his grasp as he dived over the goal line in a match-turning moment.
Grammar then swept on attack and scored under the posts after Tauranga failed to clear their 22 and soon after kicked a penalty to lead 18-7.
Tauranga showed their attacking potential with a top-class team try, finished in the corner by winger Taylor Kite. But that good work was wasted minutes later by another poor defensive kick that gifted Grammar seven points and a 25-12 lead.
Inspired by skipper Kelly, Tauranga finished strongest with prop Peter Seeling crashing over for a late try that on the run of play deserved to be the match-winner.
But it was not to be.
Returning Tauranga coach Dan Goodwin was disappointed with the loss after his team beat King's College in Auckland in another traditional fixture.
"This was a big game for us. They are very important for us these traditional fixtures and it is a good gauge of where we are at against some of the better Auckland schools," he said.
"There was no lack of will in there but just a bit of decision-making at some stages of the game that let us down. We will learn a lot from it and need to follow the systems we have in place. There were some pretty valuable lessons out of that I think."
Goodwin and co-coach Richard Apanui have half their squad new to First XV rugby this year. They will now refocus on the Chiefs Cup beginning against St Paul's Collegiate in Hamilton on Saturday with the Super 8 to follow later in the season.
'We then go Hamilton Boys' and Manurewa so it doesn't get any easier. It is a tight schedule from here on in but it is good for us," Goodwin said.