Breaking All the Rules

What sorts of things work in writing middle grade? What isn't allowed? There are certain rules writers hear about at every phase of the creative process. But there always seem to be exceptions to those rules. Here's one I've seen successfully broken again and again of late:

Too Many Adults:
Middle-grade novels need to be populated with kids. Throw too many adults in, or worse, let too many adults start filing up scenes, and you'll lose your audience.

THE ABSOLUTE VALUE OF MIKE - Katherine Erskine
Apart from a Romanian orphan mentioned only in conversation (and seen in a brief video), Mike is the only kid in this story. His world is populated by Mike's genius but clueless father, his Great Uncle Poppy and Great Aunt Moo, and other adult characters, like Past, a homeless man.

THE GREAT UNEXPECTED - Sharon Creech
Best friends Naomi and Lizzie meet the strange Finn boy when he falls out of a tree. Much of the story alternates between the three children and two scheming elderly women half a world away. The story lines eventually intersect, but not before Lizzie and Naomi spend much of the story in a world of adults.

SPARROW ROAD - Sheila O'Connor
Twelve-year-old Raine O'Rourke follows her mother to Sparrow Road, an artists' colony where her mother has taken a job for the summer. Raine learns how art can both teach and heal and is essentially the only child character in the story.

HORTON HALFPOTT: OR, THE FIENDISH MYSTERY OF SMUGWICK MANOR; OR, THE LOOSENING OF M'LADY LUGGERTUCK'S CORSET - Tom Angleberger
Horton, the lowly kitchen boy, is accused of stealing the Lump, the Luggertuck family heirloom. While he has young servant friends, Bump, Blemish, and Blight, much of the story is peopled with adults: the self-centered M'Lady Luggertuck and her dastardly, dim-witted son, Luther (who though not an adult is older than the typical mid-grade character); the wooden-spoon wielding cook, Miss Neversley; Old Crotty, the lady's maid; Portnoy St. Pomfrey, the foul-breathed detective; and a band of land-locked pirates, to name a few. (I listened to this one on CD and enjoyed it so much, I read it again with my fourth grader.)

Can you think of other books that break this Too Many Adults rule? What about other mid-grade writing rules that have been successfully broken?