Hastings lost 24-22 to Orewa in pool play but they had a gobsmacking 22-6 lead before the losing finalists clawed their way back to victory.
"We made a couple of errors which let them back into the game and they got a roll on from there," Foster said.
The champions emerged from their pool in third place after they had on Monday succumbed 22-16 to Rotorua, who were on the path of revenge after losing in last year's 12-6 final loss to Hastings.
On Wednesday Hastings won three of four games but stumbled against Orewa in their final round-robin match.
Not wanting to sound like they were looking for excuses, he said some penalties went against Hastings on the foundation of those self-imposed errors.
"We went back to the motel that night knowing we needed to tighten up our defence.
"We knew we could attack but it was the defence we needed to work on."
Talk is often cheap but the sermon from Foster and Tamati struck a chord as the schoolboys delivered with aplomb.
Hastings started the last three games with a 44-0 walloping in the quarterfinals before making Rotorua swallow more bitter medicine with a 10-0 semifinal victory.
"What was pleasing for Kevin Tamati and I was that the boys didn't let anyone cross the line on the last day."
Foster said it was ironic that his "rugby kids" came to Hastings Intermediate having never played rugby league.
"There's no rugby league pathway in Hawke's Bay for these youngsters," he said.
"Some people don't realise how challenging it can be to train a bunch of children to play rugby league so they can go on to become talented enough in New Zealand to become champions."
Part of the team discipline was lights and TV out by 9pm and the adult pair impressing on the youngsters that they weren't there to socialise but do their school and region proud because every other school saw them walking around with a target on their backs as defending champions.
After etching the school's name again on the trophy, Foster said the players' curfew was eased to a 10.30pm lights out.
Thompson, Psyris Aranui, Iraia Edwards and Derek Honotapu were the Hastings players to make the tourney team.
However, Foster said Bibby ("a tackling machine") and Manaia Lambert ("an elusive winger with his stepping and pace") also ought to have been included.
"You know, Manaia reminds me of Jason Robinson," he said, referring to the former English international.