I think the first time I heard this expression was in the Enid Blyton's St. Clare series. The girls at the boarding school where all the stories took place would realize their mistakes and turn over a new leaf, apparently. I loved reading about English boarding school life, though I now find the system rather appalling. I would never be able to send my own kids away to school.
My favorite thing about the books was that in each one they would cobble together treats and have a secret, illegal, midnight feast, complete with roasted sausages. This seemed the height of fun to me, but the closest we could come to it was raiding the fridge during a slumber party.
Because of my early immersion in British boarding school life, the Harry Potter series, with its head boy and head girl, was a familiar world.
I've already turned over the new leaf of keeping my house clean, but yesterday there was no showing and cookies were made with Grandma and now it's a bit of a mess again. So, I guess that's what the "again" is about!