I second the recommendations of John Scalzi and Timothy Zahn for good action adventure that is teen-accessible. I'd suggest, if you're looking for something with some "moral issues" in it, that you try "Little Brother" by Corey Doctorow, or "Hunger Games" by Suzanne Collins. "Feed" is another strong book with political overtones. Lastly, if you want some aspects of computerized battle and war-games, you try "Epic" and "Saga" by Conor Kostick.

Another author whom I've found very interesting, but is probably not accessible to most teens, is Julie Czerneda. Her books are very strong in the biological sciences, and do a very good job of presenting truly "alien" aliens, and have some very interesting twists in the plot.

Dragon and thief : a Dragonback adventure / Timothy Zahn This is a series of 5 books. 14 year old orphan, Jack, is on the run for a crime he didn't commit. He finds himself hiding out on an unoccupied planet, his only companion an artificial intelligence programmed with the personality of his con man uncle Virge. Jack unexpectedly meets Draycos, a dragon-like K'da (an alien species) , sole survivor of an advance team of K'da and due to circumstance the two must form a symbiotic relationship in order to survive and find out how to clear Jack's name and get the police off his back. It is generally catalogued as Adult Science Fiction, but I think they are fantastic for teens who enjoy these types of exotic sci-fi stories.

In the most general sense that it involves someone joining a military force, going through training, learning about life, the universe, and everything, and making and losing friends and enemies, I would recommend John Scalzi's "Old man's war" and it's sequels, of which there are 3-4 total. It's kind of like ender's game, except that ender is a kid and the main character here is a septagenarian(Spelling?) who thinks he's going out to fight off alien wimps, since the military he joins recruits exclusively senior citizens!

There is another series, called Orphanage, I believe. It's about a man named Jason Wander, who, in the first book, is 17 and becomes orphaned in a war that planet earth was completely unprepared for. The remainder of the series is his joining the military and his adventures in said military forces. It's written by Robert Buettner.

I've only read one of the "Ender" books so this may not be quite what you're looking for, but what about Terry Pratchett's "Johnny Maxwell" books? The first one, Only You Can Save Mankind, opens with 12-year-old Johnny blithely slaughtering aliens in a video game, only to be contacted by actual aliens - who want to surrender... There's a fair amount of humor (this is Pratchett, after all) but there are some serious issues as well, including details of the home lives of Johnny's friends.

The other books don't deal with aliens, though they each have fantasy-and/or-SF elements: Johnny and the Dead has Johnny and friends trying to save the local cemetery's ghosts from having their turf developed, and Johnny and the Bomb has a time-travel element.

The books seem to be written for early teens, language-wise. They do touch on some rather harsh realities in among the humor and adventure; one of Johnny's friends comes from an abusive household, for example, and each of the main characters has their own troubles. The emphasis is on forging friendships, doing the right thing, and dealing with reality (yes, even when it involves aliens or ghosts {wry grin}), and the triumphs tend to be bitter-sweet. I enjoyed them a lot.

I'm in a YA Lit class at the moment, and Ender's Game is paired with Feed / by MT Anderson.

If you really liked the Science fiction end of things, Isaac Asimov has some great reads, and some of his books have been made into movies (I, Robot, Bicentennial man.) The guy practically created the genre.

Again with the general military theme: Robert Heinlein's "Space Cadet" was interesting and kind of fun. The Academy is located in Colorado, there is another part of the Academy in space. A semester (I think( in the asteroid belt and a first postin on Venus. I liked this as a kid in the late 70s.

I second Heinlein, and Feed by Anderson. Look at the books by William Sleator, though they are much shorter, but they feature a lot of teens being the smartest in the room to solve a technical and a moral problem in a science fiction setting. The White Mountains by Christopher, if they can take a little darker and give up the action, but like the thematic elements. Orson Scott Card also writes some urban based fantasy, and I especially liked Magic Street.

What part of it did you like? The kids in science fiction? The space opera? The moral issues? The twist ending? The straight science fiction?

For an Ender fan, I'd recommend Warchild by Karin Lowachee and it's sequels. Orphan boy trained to fight, space pirates, aliens at war with humans...

Patricia C Wrede's newest, Thirteenth Child, strongly reminded me of Orson Scott Card's writings, although more his other works than the Ender series. It contains the element of special kids, sibling relationships among unusual siblings, exploration of territories, alternate history, and unknown lifeforms.

Megan Whalen Turner's Thief series also contain military aspects, particularly the second and third books.

For space opera, I'd reccomend something by Lois McMaster Bujold. I haven't read all of them (yet), but the ones I have star Miles, a physically disabled soldier kept largely a secret in his planet's security system because he's a tactical genius. The Warrior's Apprentice is one of my favorites so far, in which he assembles a functional, loyal, and illegal army more or less by accident. One of my favorite characters to date.

Dear Bookpeople:

Also not quite like Ender's Game, as the protagonist is an adult emeshed in his career, but

The Space Merchants by Pohl and Kornbluth

has space action, moral choices, etc.

The Moon is a Harsh Mistress by Heinlein might also work.

Robert in San Diego

So many good tips here. Just to add my own: Zoe´s tale by John Scalzi and the first Vorkosigan books by Lois McMaster Bujold (The Warrior´s apprentice, Cetaganda, The Vor game)

I would peg Sherwood Smith's Inda series as similar, except it is fantasy instead of sci-fi. It features a boy who goes to military academy and turns out to be something of a military genius, and then there is the banishment, pirates, ghosts, politics, sword-fighting, etc. I found it to be quite engaging.

Dear Bookpeople:

Not a series, but I can recommend

Babel 17 by Samuel P. Delany

Strong female protagonist, interstellar space warfare, information theory (!) and lots of action. It's short, and I don't believe there was a sequall, so It might be a bit adult, but it'll hold even a voracious, high-speed high-comprehension reader like my nephew for a day.