The win gave the team back-to-back league titles; an incredible achievement for boss Matthew Etherington, assistant Simon Davies and the under 18 players.
Two goals in the last 10 minutes of the game on Saturday, February 13, gave Posh a 2-1 win in their final fixture of the league campaign at the idverde Training Ground.
A win for Southend would have kept their title race alive - as they had games in hand over Posh - and they broke the deadlock on the stroke of half time with a close-range finish from a free kick.
The Shrimpers keeper denied two efforts from Ricky-Jade Jones, and Connor Peters and Charlie O'Connell missed out by small margins, until O'Connell tapped in an equaliser from close-range.
In the end, it was substitute Adler Nascimento who found the winner for Posh. He came on for Jones in the 75th minute and found the back of the net with only a few minutes play left.
Speaking after the game, Matthew Etherington, Posh youth team manager, praised the togetherness and resilience of the team.