It was too little too late for Posh Under 18s who saw three second half goals serve as consolation as Barnsley claimed the points in South Yorkshire.
A Toby Smith brace inspired Barnsley to a first half rout which saw them five clear at the interval, but the tale that football is a game of two halves showed as Posh came out of the blocks and found the net on three occasions to lessen the deficit.
Jimmy Unwin resorted back to his favoured line-up which saw three alterations from last time out. Noah Freeman, Eddie Fox and Joe Davies all returned to the eleven after stints with the Under 21s in midweek.
Like last weekend, Posh found themselves behind early doors. This time around it was Smith who curled an effort past Ben Westcott and into the top corner.
Six minutes later and Smith had his brace with a similar finish to his first which saw the forward pick out the top corner and double the hosts lead.
In the middle of the celebrations Posh were forced into an early substitution after Luke Gilbert replaced Lucca Mendonca.
Barnsley added a third on 15 minutes when Elliott Probert arrowed a shot into the bottom corner to provide Posh with a difficult task at hand.
The task would only become more difficult when Keiran Graham bundled the ball home for a fourth on the half-hour mark.
Posh then capitalised on a rare attack when Bolu Shofwoke drove forward and found Patryk SYKUT who fired past Jake Andrassy.
Barnsley hit back with an instant response, Luke Alker was on hand to add a fifth with what was the hosts fifth opportunity on goal.
Graham rounded off a clinical first half with his second of the afternoon when his effort from distance looped over Westcott and into the back of the net.
Barnsley set off how they concluded the opening period, Smith was denied of his hattrick by the woodwork on 53 minutes.
Joe DAVIES – like he did with the Under 21s – found the back of the net in Oakwell, this time a superb strike from distance rifled into the bottom corner.
From then, Posh were on top and resorted to the bench for attacking options; Benjy McWilliams-Marcano entered the field and made an immediate impact down the right.
Posh were rewarded for their pressure on 79 minutes when Bolu SHOFOWOKE tapped home for a simple finish.
With almost the last kick of the afternoon, Fabian CLAXTON added a fourth after a set-piece routine caused issues in the hosts penalty area which saw the defender pounce and add a fourth for Posh.
Posh: Westcott, Freeman, Claxton, Mendonca (sub Gilbert, 12min), Gbajumo (sub Fitzpatrick, 45min), Holley, Fox (sub Unwin, 45min), Changunda © (sub McWilliams-Marcano, 71min), Sykut (sub Simmons, 83min), Davies, Shofowoke. Unused subs: none.
Half-Time: Barnsley Under 18s 6-1 Posh Under 18s
Full Time: Barnsley Under 18s 6-4 Posh Under 18s