I should probably start my own “name this book” thread but hopefully I can get an answer here. If I do, it will be my answer to the OP as well. I read this series when I was about 12. Before that I had only read fantasy like LotR or Narnia. It was several books about an alien invasion. I believe it was an English author. The main characters were children. I think the title of one of the books had the word mountain in it. They are pretty famous because I saw that BBC did an adaption many years ago that I caught part of on PBS. I wanted to get these books for someone but couldn’t remember the name of the books or the author. Next time I will try to be more vague.
Crisis on Conshelf Ten by Monica Hughes was one of my very favorite sci-fi novels when I was a kid. I still have a battered copy that I read again every few years. 
Anne McCaffrey’s Harper Hall of Pern series.
Monica Hughes’ Keeper of the Isis Light (think there’s a few sequals as well)
Madeleine L’Engle’s “A Wrinkle in Time” series.
Garth Nix’s “Sabriel” series
That’s all I can think of right now. I’ll post more if/when I remember them.
We’re currently in the Golden Age of Young Adult fantasy and science fiction, with more books being published each year than in the entire decade of the 50s combined.
However much you might have liked Heinlein, Asimov or Norton when you were a kid, why not let the kids of today discover the books of today and find their own favorites to pass down to their children?
The definitive listing can be found at the amazing SciFan.com. Their theme called Young Adult - books for teens lists 3471 titles and has an alphabetical listing of writers and series. It’s presented in chronological order, so the best way to approach it is by scrolling down to the bottom of the page and clicking on the Last link and working backward.
Each book has links to buy it, and also has a description. Taking one at random.
Blue Roan Child, The, by Jamieson Findlay
They have other Themes for books for younger children.
Because SciFan is so all-encompassing, it’s not really the first place to search, but an indispensible backup for getting information on books listed elsewhere.
To narrow it down a bit, here are some other sites to check out.
The Golden Duck Awards gives out three awards annually, for picture book, middle grades, and the Hal Clement Award for Young Adult books.
The Aurealis Award does the same thing for Australian authors. Their books are listed and can be checked for availability on SciFan.
Here’s a very different discussion, maybe of more interest to girls, from the Romantic SF page. Girls now make up as large or larger a fraction of the young f&sf audience than boys do, remember.
Library pages giving YA lists:
http://www.mppl.lib.in.us/yaSCIFI.htm
http://www.menashalibrary.org/youth/YAbk_scifi.html
http://www.evanston.lib.il.us/library/bibliographies/humorous_fantasies.html
A Possible Worlds page.
The Edgar Rice Burroughs “John Carter” books are worth considering. Depending on who painted the dust jackets, the sexuality is fairly restrained.
Depending on her attitude towards comics, maybe some anthologies of “Buck Rogers” and “Flash Gordon.”
Are you thinking of John Christopher (The Tripods)? Boys Life magazine serialized some of this series in “graphic novel” format when I was a kid.
If you want some slightly more comtemporary kid’s SF, then there are plenty of choices.
Anything by William Sleator is good. Try Interstellar Pig or Marco’s Millions. Aliens Ate My Homework by Bruce Coville, Outernet by Steve Barlow and Steve Skidmore or The Giver by Lois Lowry would also be good additions. They’re all fine for 5th or 6th graders.
Douglas Adams is great read for slightly older teens. Every book he ever wrote. Ender’s Game by Orson Scott Card, The Postman by David Brin.
The Ear, the Eye and the Arm or The House of the Scorpion by Nancy Farmer, Virtual World by Chris Westwood, Francine
If you want some slightly more comtemporary kid’s SF, then there are plenty of choices.
Anything by William Sleator is good. Try Interstellar Pig or Marco’s Millions. Aliens Ate My Homework by Bruce Coville, Outernet by Steve Barlow and Steve Skidmore or The Giver by Lois Lowry would also be good additions. They’re all fine for 5th or 6th graders.
Douglas Adams is great read for slightly older teens. Every book he ever wrote. Ender’s Game by Orson Scott Card, The Postman by David Brin, The Ear, the Eye and the Arm or The House of the Scorpion by Nancy Farmer, Virtual World by Chris Westwood or After by Francine Prose should be good for older kids.
Hitchhiker’s Guide to the Galaxy? I was about 14-15 when I first read it.
Tom Swift books.
Gotta have Tom Swift!
{Of course I’m dating myself}
Jeez, 46 minutes. What took you so long. This is the Straight Dope, I expect immediate answers! 
That’s it. Thank you very much. These are the books that got me started on a lifetime of Science fiction. Look at the reviews at that Amazon link. Agonist a lot of the books that have been recommended I think would be better suited for early teens rather than the pre-teen group you are asking about. I would put the Tripod books at the top of your list.
I would also put The Dark is Rising on the list. More Fantasy than Science Fiction but it is suited for the audience and quite good.
So that’s what that was!
Jeez, I was sure you were talking about Escape To Witch Mountain! But since you weren’t, I’ll mention it anyway. It’s closer to what they’re reading in third and forth grade these days, but some slower readers will appreciate it.
Amazon’s age ranges are much too high for what kids around here are expected to be able to read. Granted, I think that says more about how quickly the schools are pushing them than the books themselves, but WhyKid’s due to read To Kill a Mockingbird after the holidays - at age 11. Many of the “Young Adult” listings by Amazon are in our fourth grade classrooms. (I was asked to make a book list for the sixth grade this year, and had to push my expectations up about 3 notches, because what I was recommending, they’d already covered in previous years.) YMMV
*The Giver * is fantastic, but probably already in their library.
Plus the rest of the two series:
The Ender series:
[ul]
[li]Speaker for the Dead[/li][li]Xenocide[/li][li]Children of the Mind[/li][/ul] [ul]
[li]Ender’s Shadow[/li][li]Shadow of the Hegemon[/li][li]Shadow Puppets[/li][li]And when released next year… Shadow of the Giant[/li][/ul]
And First Meetings: In the Enderverse which is four short stories related to the books.
Heh. I was coming into the thread to recommend the Tripod books.
Also, Arthur C. Clarke is a must-have, as is Ray Bradbury. At a minimum, the well-stocked middle-school library should have 2001, Childhood’s End, The Martian Chronicles, and Fahrenheit 451.
Seems to me that SF fans are often pretty advanced readers; I see no reason why the non-sex-based adult SF shouldn’t be there. Consider adding Red Mars, which doesn’t have much sex in it. Definitely have The Golden Compass and the followup books (which are also borderline fantasy).
And I LOVED the “Danny Dunn” books when I was a kid, like second grade or so. They were my absolute favorite.
I’m vaguely up-to-date on modern kids’ fantasy, but I’m not so good on SF.
Daniel
Even without the sex, Vorkosigan is way too advanced for preteens. I’d be careful with the Pullman as well. Most kids aren’t going to read Garth Nix at 10 either.
Actually, I noticed that they already have the Pullman trilogy in their collection. Surprised me quite a bit considering the whole “no sex” thing, but it was pretty essential to the story. And I guess it was mostly off-screen, too, now that I think about it.
I’ve seen some pretty sophisticated stuff, as well as some pretty mindless entertainment there. Since I’m not sure myself what qualifies, I’ll just be passing the suggestions on to my friend.
I knew you guys would come through for me! Any other ideas?
The Sabriel series is fantasy. There’s a bit of technology but it’s WWI era, not sci fi. Great wonderful beautiful series, though- I just read it last week.
Monica Hughes wrote quite a lot of preteen sf. There’s the aformentioned Keeper series, and Crisis, but she also wrote Devil On My Back, Ring-Rise Ring Set, The Promise, Sandwriter, and a bunch more I can’t think of right now. Good books.
I just checked my email. My librarian friend sent me this comment:
Woohoo! Sleator is good!