The hosts were on the wrong end of an 11-6 penalty count and Ioasa said his players struggled with interpretations.
The hosts' frustration was evident in the final minutes when substitute prop Corban Kireka was red carded for punching.
Hastings Boys' High School will host Palmerston North Boys' High School on August 18 in a match they must win to keep their hopes of retaining their Hurricanes region title alive, as well as the opportunity to retain their National Top Four title.
"The positive is we've got a fortnight to prepare for that game," Ioasa added.
Hamilton Boys' produced their best performance of the season, an ideal 18th birthday present for centre Josh Calvert.
Halfback Cortez-Lee Ratima was classy and winger Leweni Mocekaca made the most of his limited opportunities in the difficult conditions.
Captain Tom Martin and fellow flanker Jake Ross were workaholics.
"Jake might not be the biggest of flankers around but there wouldn't be any with a bigger heart than his," delighted Hamilton coach Nigel Hotham said.
Like Ioasa, Hotham, had four regular starters from his 2017 squad back and there was plenty of talk in his camp about gaining revenge for last year's 25-17 loss in the national Top Four final.
On Saturday's display Hotham's team will have to start favourites against Napier Boys' High School next week.
"I had a coffee with Brendon [NBHS coach Ratcliffe] this morning. Next week we go to war," Hotham added.
Openside flanker Connor Wadley had a huge workrate for Hastings while substitute forward Ngiyaani Waters provided quality impact off the bench.
Second five-eighth Jordan Thompson-Dunn was the pick of the Hastings backs.
Scorers:
Hamilton Boys' High School 26 (Cortez-Lee Ratima, Carlos Karaitiana, Tom Martin, Leweni Mocekaca tries; Demante Murray 3 cons), Hastings Boys' High School 0. HT: 12-0.
