6th grade = 11/12 years old, right? So how about sections of one of the Real Classics, The Odyssey? In translation, of course! 
Or an age-appropriate extract from the Thousand and One Nights?
And another Discworld YA, The Amazing Maurice and his Educated Rodents.
I forgot about A Wrinkle in Time! Definitely put that one on the list
Might be too “young” for your class, but I would vote for The Last Unicorn by Peter Beagle.
I adored Watership Down but there were parts of it that I thought were very sad.
Also, it’s hard to find in print sometimes, but Sheri Tepper’s True Game series. Juvenile fiction before she got kind of heavy handed in the feminist messages. It’s extremely good - seems like only a fun romp until you get to the end and realize it’s a moral tale.
Enough material here with the runners-up for the rest of your teaching career.
http://www.ala.org/alsc/awardsgrants/bookmedia/newberymedal/newberyhonors/newberymedal#20s
Word of caution: these are selected by adults. Some kids like, some they find deadly boring. Consult your school" librarian/media specialist.
P-man, MLS
For the love of god, please don’t make any kids read “I, Juan de Pareja”!
I have never in my 25 years as a librarian seen anyone check that book out. When I built a school library from scratch 5 years ago I did not include it in the opening day collection.
The Golden Goblet always went over well with my mom’s sixth graders. I read it as an adult, and quite enjoyed it.
L. M. Boston – The Green Knowe series
The Children of Green Knowe
The Treasure of Green Knowe
The River at Green Knowe
A Stranger at Green Knowe
An Enemy at Green Knowe
The Stones of Green Knowe
Daniel Pinkwater – The Snarkout Boys books
The Snarkout Boys and the Avocado of Death
The Snarkout Boys and the Baconsburg Horror
Eoin Colfer – The Artemis Fowl series
Artemis Fowl
Artemis Fowl: The Arctic Incident
Artemis Fowl: The Eternity Code
Artemis Fowl: The Opal Deception
Artemis Fowl: The Lost Colony
Artemis Fowl: The Time Paradox
Artemis Fowl: The Atlantic Complex
Artemis Fowl: The Last Guardian
Lewis Carroll – The Alice books
Alice’s Adventures in Wonderland
Through the Looking Glass
When I was that age, I was pretty much mainlining Diana Wynne Jones. A lot of times, fantasy doesn’t get no respect, because it’s fantasy, but a lot of her books grapple with good solid pre-teen issues - dealing with stepfamilies (Ogre Downstairs), bullying (Dogsbody, Eight days of Luke) growing into responsibility (Power of Three) and the difference between first impressions and actual reality (nearly all of them)
Margaret Mahy is a wonderful New Zealand kids/YA writer who wrote both excellent fantasy fantasy and mainstream (but, warning, does include sexual issues sometimes, so not all appropriate for grade 6’s)
I think a lot of folks are missing the “something significant” part. Artemis Fowl is a lot of fun, but that’s some serious fluff. It’d be hard to have a good discussion about character re: that book, for example.
Fantasy, though, has richer veins to mine. What about A Wizard of Earthsea? Or the winners of The Andre Norton Award given out to excellent YA SF/F? (From that list, I totally recommend **The Girl Who Circumnavigated Fairyland In A Ship of Her Own Making).
She’s stuck all alone on an island after her little brother is torn apart by wild dogs. And it just keeps getting worse except for this weird and ultimately depressing relationship she forms with one of those dogs. Not a life I’d envy I guess.
Since somebody mentioned Diana Wynne Jones, let me recommend these of her books:
Diane Wynne Jones – The Chronicles of Chrestomanci
Charmed Life
The Magicians of Caprona
Witch Week
The Lives of Christopher Chant
Mixed Magics
Conrad’s Fate
The Pinhoe Egg
Diana Wynne Jones – The Castle series
Howl’s Moving Castle
Castle in the Air
House of Many Ways
and since somebody mentioned The Wizard of Earthsea, let me mention the whole series:
Ursula K. Le Guin – The Earthsea books
A Wizard of Earthsea
The Tombs of Atuan
The Farthest Shore
Tehanu
Tales from Earthsea
The Other Wind
and then of course The Giver is part of a series:
Lois Lowry – The Giver Quartet
The Giver
Gathering Blue
Messenger
Son
In the Andre Norton Award link given by the Left Hand of Dorkness, it mentions Ysabeau S. Wilce, so I’ll mention her Flora series:
Ysabeau S. Wilce – The Flora Fyrdraaca/Califa books
Flora Segunda
Flora’s Dare
Flora’s Fury
Prophecies, Libels and Dreams
Those are really for someone older than sixth grade though, and the last of those books is definitely for adults.
Someone correct me if I’m wrong (I only listened to the audiobook version), but The Graveyard Book by Neil Gaiman might be appropriate for a sixth grader. Some people die and most of the cast are ghosts, but I don’t recall it being particularly depressing or adult in nature.
Graveyard Book is excellent, and certainly appropriate for 6th grade.
Great Brain
What about the L.M.Montgomery “Anne of Green Gables” series?
American classic…
The Little House books by Laura Ingalls Wilder. The first one would be too young for 6th graders, but some of the later books are perfect.
You get literature and American history in one. It is the life that LIW actually lived.
And the kids should understand our pioneer origins and what life was like on the frontier in the second half of the 19th century.
We read the first two Anne of Green Gables books in 6th grade. Sad things happen in them, but the books, overall, are not sad.
The only other thing I remember reading for school was “Black Beauty.” IIRC, it’s about horrible things happening to a horse.