The victory at Old Trafford ensured an immediate return to the second tier for a side that were relegated last season, plus the former Iron midfielder Grant McCann who moved to Posh in the summer after turning down a new contract with the Iron.
He got on the scoresheet for Darren Ferguson's side, scoring the third goal - direct from a free-kick - after Tommy Rowe and Craig Mackail-Smith had put them in command.
They are the third team to move up to the Championship this summer, behind champions Brighton & Hove Albion and Southampton.The Iron, Sheffield United and Preston North End drop down to replace them.
Chesterfield, Bury, Wycombe have been promoted to League One. Dropping down to League Two in their place are Dagenham, Bristol Rovers, Plymouth Argyle and Swindon Town.
Confirmed League One teams for 2011/12: Bournemouth, Brentford, Bury, Carlisle United, Colchester United, Charlton Athletic, Exeter City, Hartlepool United, Huddersfield Town, Leyton Orient, Milton Keynes Dons, Notts County, Oldham Athletic, Preston North End, Rochdale, Scunthorpe United, Sheffield Wednesday, Sheffield United, Tranmere Rovers, Chesterfield, Stevenage, Walsall, Wycombe Wanderers, Yeovil Town.