Second five-eighth Luke Ale opened Hamilton's account with a try five minutes into the second half. It followed 10 phases of sustained pressure.
First five-eighth Taha Kemara added the conversion. A successful penalty kick from Kemara two minutes later had Hamilton ahead for the first time in the match at 10-8.
Ah Kiong replied with a penalty in the 45th minute. Considering the atrocious weather conditions it was a mighty effort from the composed Hastings side which had no passengers.
They will meet King's College of Auckland in Sunday's final. King's College beat Nelson College 49-17 in their semifinal.
Meanwhile it couldn't have been more appropriate ... former Magpies Tivaini Fomai and James Rosenberg were among the first to congratulate Hastings Boys' High School after they won the under-15 title.
Rosenberg and Fomai were in the last HBHS side to win the title in 2000 in Wellington and they were the guest speakers at the jersey presentation function for this season's team last weekend.
"An awesome gesture and a great day," the manager of this season's team and the 2000 side, Quentin Crawford, said after Hastings beat defending champions Hamilton Boys' High School 38-5 after leading 17-0 at halftime.
Classy winger Alex Dickey scored three tries. Powerful loosehead prop A one Lolofie, blindside flanker Bonham Edwards and fullback Tamati Te Pou scored one each. First five-eighth Kooby Deacon kicked four conversions.
"Matt [coach Robertson] did his homework. We played our normal game, stuck to our systems and processes and it all paid off," Crawford said.
Part of Robertson's homework was identifying how big a threat one of the Hamilton midfielders could be if given too much freedom. Hastings second five-eighth Bryson Ioane did an outstanding defensive job on that player.
Lolofie, fellow prop Naverone Te Rini and hooker and vice-captain Vikta Tevita impressed with their devastating carries. Openside flanker TJ Vekene and No 8 Ned Saifiti were full of authority at the breakdowns.
Crawford agreed that it took his side 10 minutes to get into the match.
"Both teams were pretty tentative as you would expect on a big occasion like this. But we had a lot of support and that paid off big time.
"There were students and teachers from school. There were parents and grand children of our players. To see the smiles on the boys faces at the final whistle ... you can't buy that. We've been together since Marsh so get the reward at the end of the season is pretty special," Crawford said before heading off to give the lads an eagerly awaited visit to a fast food outlet.
Three of the squad, midfielder Tali Ioasa a son of the school's first XV coach and former New Zealand Sevens star Tafai Ioasa, winger Phortre Gurneck and loosie D Angelo Etika, will be eligible for next year's title defence in Christchurch.
Robertson and Crawford are likely to take the school's under-14 side next year.
Palmerston North Boys' High School pipped hosts Napier Boys High School 13-11 courtesy of a penalty kick off the last play of the game to secure third place. Napier trailed 10-0 at halftime but as they did in the previous day's semifinal loss to Hamilton Boys' High School staged a courageous comeback and led 11-10 until the final second.
Winger Kade Barham, a son of former Magpies midfield back Andy Barham, scored the hosts only try and winger Jake Evans, a son of former New Zealand under-19 team and Magpies lock Rob Evans, kicked two penalties.
Napier's loose forward trio of captain Max Ratcliffe, fellow flanker Gus Brown and powerful No 8 Telea Faumui again functioned well.
Other playoff games saw St Bede's College beat Rotorua Boys' High School 22-19 to secure fifth place, New Plymouth Boys' High School pipped Auckland Grammar to finish seventh, Wesley College beat Tauranga Boys College 29-15 in the battle for ninth, Mount Albert Grammar School beat Westlake Boys' High School 31-19 to finish 11th, Christchurch Boys' High School pipped Otago Boys' High School 20-18 to finish 13th and Wellington College beat St Pat's Town of Wellington by default to finish 15th.