Reflecting on a 39-28 success over Capital rivals Stewart's-Melville, Armstrong pointed to the fact that most of the side will now move on to university and many will not play the game after they have graduated.
He said: "Ross Turner, our captain, is in the Scotland under-18 set up so he has been earmarked for something but, for most of these boys, that will be their highlight, so it's a good way to go.
"That's the first time we've managed to win the trophy and it means a heck of a lot to the boys and to us."
Armstrong's fellow coach Duncan Harrison summed up the quality of a game that produced flashes of outstanding play from both sides and commitment levels that never dipped.
"It was a great advert for school rugby," he said. "It was a game where either team could have frozen but both went out and played. It ebbed and flowed. Both teams didn't give an inch."
And praising his players, who had started as underdogs for the second successive round, and had then fallen behind to an early onslaught from their opponents, he added: "As a group, their confidence has grown a massive amount and their belief has also grown.
"This is a group that has been working together for a while now and a lot of them will be leaving us so it's fitting.
"But there's a little bit more than just huff and puff and self belief, there's some pretty good players out there. Our nine and our ten were outstanding."
StewMel dominated the opening quarter and deservedly led 11-0 after 17 minutes, with full back Rory Paterson racing in for the opening try of the night and winger Scott Cook thumping over two long-range penalties.
However, Fettes remained calm and stand-off Kit Gammell clawed back three points with a penalty before blasting the game wide open when he raced on to a kick ahead and profited from a fumble in the Stew-Mel defence to gather and touch down for a try that he also converted.
The Fettes revival was completed shortly before the break when Mark Agnew forced his way over from close range to finish the first-half scoring.
Player of the match Sam Hunt extended his side's lead shortly after the restart when he picked up at a ruck 45 metres out and scurried over.
Cook trimmed the gap with another penalty but the contest looked over when Gammell pounced for an interception try then added the conversion and also slotted the kick after Rory Daly had bagged another try to extend the Fettes advantage to 20 points.
To Stew-Mel's credit, they responded with a wave of attacks and the pressure paid off when they earned a penalty ten metres from the opposition line – flanker Chris Graves was dispatched to the sin bin for the offence – and George Turner burrowed over for a score, which Cook converted from the touchline.
Then, when Craig O'Donnell powered in for another converted try with eight minutes to play, hopes were raised among the vociferous Stew-Mel support.
However, the trophy was destined for Fettes and centre Chris Giffin sealed the silverware when he won a kick and chase for his side's sixth try.
In the opposition camp, coach Chris Spence praised his players for the way they battled back into a contest that appeared to be lost. He said: "We had a good start, but they came back at us well. I thought at one stage we had just gone too far behind but the boys showed a lot of spirit and I thought they showed a depth of character to push it back to one score again. I have a lot of boys with talent in the team and collectively they were very good." However, he conceded their opponents were worthy winners, adding: "Overall, I have no complaints. I thought Fettes played very well, they played some good rugby, were defensively very sound and dangerous in attack.
"It was an excellent game of rugby. It's difficult to say what the difference between the sides was but we made one or two silly mistakes at crucial times and they took advantage of them."
Stewart's-Melville: Tries: Paterson, Turner, O'Donnell; Conversions: Cook (2); Penalties: Cook (3).
Fettes: Tries: Gammell (2), Agnew, Hunt, Daly, Giffin; Conversions: Gammell (3); Penalty: Gammell.
Stewart's-Melville: R Paterson, S Cook, S Murchie, M Crawley, M Doneghan, R Grant, G Milne, M Campbell, S Shaw, G Turner, R MacLeod, C O'Donnell, N McGlinchey, B Johnstone, J Kynoch. Sub: A Wilson
Fettes: R Daly, R Black, J Greer, S Daly, C Giffin, K Gammell, S Hunt, R Turner, S Cooper, B McDonald, B Morrison, J Grant, P Chamberlain, C Graves, M Agnew. Subs: S Tosh, M Birrell, C Biagi.