Surely you can’t be serious. It’s a kissing book!
How about Machiavelli’s “The Prince” or Sun Tzu’s “The Art Of War”?
Has my vote. Granted, you might get lynched by any fundamentalist kid’s parents, but that’s just the risk one takes in pursuit of what is right.
I love how Richard Adams wove in the lapine language throughout the book. So when Bigwig tells General Woundwort Silflay hraka, u embleer rah my jaw dropped because I knew what he said.
Eat shit, you stinking prince.
I’m also going to recommend Encyclopedia Brown. Not high literature, certainly, but it helps with thinking skills as you solve the mysteries.
“Thick”? You can use the dead-tree version to train Olympic weightlifters! 
Seriously, it’s a great book but it’d be a huge investment of time for even a fast reader; I’m sure there are other things the students would need to cover that year. Plus I seem to recall a scene where Van Eck phreaking revealed a story where the man had a fetish for black stockings and the woman for heirloom-quality furniture. Some parents might object.
I haven’t seen anyone recommend The Tripod Trilogy by John Christopher yet. I remember enjoying them at about that age.
IIRC, those would violate the “no oppression” requirement.
I’m trying to stick to books no one has mentioned yet. You’re right; most of the books I loved from those days are pretty tragic.
I would recommend Ray Bradbury’s “Dandelion Wine.” It’s not funny like Terry Pratchett (but a lot ess likely to get you fired than “Good Omens”). Mostly only very old people die, and no beloved characters.
We enjoyed “Cheaper By The Dozen” in sixth grade, but of course (spoiler) dad dies in the end.
The Sword and the Stone (TH White) is fun.
I’m looking at that list of Newberry honors and winners and noticed that Hatchet is on there. I listened to the audiobook a while back, and, while I don’t think it would be a super-challenging read, it’s an interesting one. It’s about a 13 year old kid whose plane crashes (pilot has a heart attack) into a lake in Canada. The kid has a few rudimentary supplies and a hatchet. The book’s basically about him surviving alone in the wilderness for 54 days until he is rescued and learning to let go of self-pity. There’s a lot of clinging to hope in the midst of despair, and one passage where he seriously contemplates suicide, but on the whole it sends a strong message about overcoming adversity.
I didn’t see the Wolves / Dido Twite series by Joan Aiken mentioned. The Wolves of Willoughby Chase is the first book, which doesn’t have a lot to do with the future books after the Dido Twite character is introduced, but it’s a good start.
I wish I had known about Diana Wynne Jones when I was in 6th grade. I don’t think she was (or is) well known in the US. Charmed Life and The Homeward Bounders are my two favorites.
And I forth or fifth The Westing Game. That was my favorite book when I was around that age.
Other people have mentioned From the From the Mixed-Up Files of Mrs. Basil E. Frankweiler by EL Konigsburg, but The View From Saturday is a good one from her too. And it’s about a group of 6th graders.
Can Roald Dahl be a choice? Matilda and Danny, the Champion of the World are pretty fun.
I don’t know if some of these are too young or not; I read most of them as an adult. My 6th grade reading was Flowers in the Attic and Steven King.
Robin McKinley’s Beauty or The Hero and The Crown (Newbery winner). Beauty is a wonderful retelling of Beauty and the Beast, and The Hero and The Crown is the story of a princess overcoming all odds to save the kingdom that doesn’t even like or appreciate her.
StG
Homecoming by Cynthia Voigt. Amazon says grade 7 up, but I’d think 11 year olds would like it. I liked it, and I’m slightly older than that. 
Spindle’s End is really good too.
Thanks for the continued suggestions. I’m going to link this thread to my supervisor so she can look through some of the ideas too.
I like all of the Flavia de Luce books, written by Alan Bradley. It’s about a girl living in 1950s England, who has a passion for science, and always seems to have a murder or so around her.
Her mother is not around, and she misses her, but it’s not a huge part of the story.
The first is A Sweetness at the Bottom of the Pie**, and I imagine it’s in your library if you want to check it out.
The Dune Series by Frank Herbert is a literary triumph. The first book is an epic adventure and it keeps getting better.
Laura Ingalls Wilder had a phenomenal memory.
She wrote about this winter 60 years after the event.
This is what USA Today had to say about how accurate she was:
‘Little House’ author right on 1880’s winter.
When Wilder writes that her father donated money for the church bell in Walnut Grove, Minnesota and historians looked through the church records, there was her father’s donation.
I’m no Laura Ingalls Wilder expert — there’s probably someone on here who knows more than I do — but I have read everything she published, every biography I know has been written about her and been to many of the LIW sites/museums.
When you’re reading the Little House books, you’re reading American history. It’s true the books were fictionalized, but the events in them actually happened. Wilder’s family really did live in log cabins and travel in a covered wagon and survive grasshopper plagues and help build the railroad and were among the first settlers of De Smet in the Dakota territory. It’s an account of the Western settlement of the United States from someone who actually lived it.
Going back a few years and might be a little too young, but I loved The Egypt Game by Zilpha Keatley Snyder when I was a kid.
The Hardy Boys
The Mad Scientist Club
The Great Brain
Funny, at first I thought you incorrectly named Born to Race, by Blanche Chenery Perrin. That was one of my favorite books as a kid, and I had completely forgotten it until now. I see I can still get it used from Amazon, and I am right now going to order it to read to my 5 yo who loves horses.
Thanks for the reminder!
And if they read The Hero and the Crown they’ll have to read The Blue Sword also.
Joan Aiken is a great suggestion - she has a series of books about Mortimer the raven that they will love.
Flavia de Luce! Yes! Awesome series. Although the one before last took a wrong turn for the series in my opinion, *Sweetness *will hook them.
I don’t know if you’re still following this thread but I found a couple of “must reads”:
Worst Class Trip Ever by Dave Barry
I Am a Genius of Unspeakable Evil and I Want to Be Your Class President by Josh Lieb
They are definitely geared toward middle-school aged readers although the Lieb one may be Not Safe for School (quite); there’s only one line that could reasonably be construed to be “dirty” but everybody, including teachers and parents, behaves atrociously. Actually, maybe it’s funnier to the middle-aged than to the middle-schooled.
I haven’t read the Barry one yet (just the first chapter on Kindle; I’m waiting for my library copy).