During a goalless first half at The Brache, Ben Stevens drew a fine save from the visitors' keeper, but a strong Barnet side also threatened with two close range headers that proved off target and a good save from Tiernan Parker required to keep the scoreline blank.
The Bees did make the breakthrough four minutes into the second half, but just after the hour mark McJannet was quickest to pounce as a Sam Beckwith free-kick came back off the bar, steering in the rebound to bring the scores level.
Twelve minutes later the U16 had his second, bagging what proved to be the winner as he pounced on a loose ball to slot past the keeper.
Lead coach Dan Walder was delighted with the way his young charges performed.
“We had an Under-15 in Jack Bateson at centre-half alongside an Under-16, Eddie Corbit, with another 16 – Josh Swan – playing left-back, when he’s normally a right-back, so the lads did exceptionally well," said Walder.
“I was really pleased with the way they played, more so than the win, because we’d done some unit specific analysis during the week and explained exactly how we wanted them to play, with freedom to break swiftly.
“So to create 27 opportunities to score and have 55 per cent possession was outstanding for such a young team.
“That’s not to say Barnet didn’t have chances of their own. They did, and they were in the game.
"But to have three of the back four not being old enough to be scholars, yet perform like that, was excellent and the win lifts us into the top half of the table.”
TOWN: Parker, Halsey, Bateson, Corbit, Swan, Pettit, Beckwith (c), McJannet, Stevens (Boorn), Kalonda (Nicolson), Lucas.
Subs: Horlick, Wedd, Boorn, Nicolson.
Goals: McJannett 61, 73
The Under-18s are next in action on Saturday morning when they entertain AFC Wimbledon in a 10.30am kick-off at The Brache, in the return of the fixture for which the venue was flipped in mid-September after the fire at Ely Way.