cahn

This isn't exactly my area--- I think I started reading science fiction in the adult section and moved backwards. Asimov's "I Robot" was one of my early favorites, and it's still a great collection of puzzle stories, but with an extra helping of weird 50s social attitudes that I'm not sure E wants to read past. (I grew into appreciating the character and emotional dimensions of fiction as I got older, but as a 6th-grader, I think, I wanted setting and worldbuilding more.)

Brandon Sanderson's "Skyward" immediately comes to mind. Teenage pilot, pet slug and artificially intelligent fighter plane start out on a space colony surrounded by aliens who keep trying to bomb them. There's a lot of Top Gun-style flight-sim action, but as the plot unrolls, she also learns more about the origins of the conflict and the possibility of peace. The second book has a ship piloted by super-cute monarchist gerbils.

Scott Westerfeld's "Leviathan" trilogy is a possibility? Steampunk World War I: the British and their living airships versus the Germans and their mechanical walkers), but way fluffier than the real war. Spunky teenage couple (heterosexual, if it matters) on opposite sides fall in love as they work together to save Europe.

I am not a huge fan of Heinlein juveniles. John Scalzi does a bunch of books that are basically "Heinlein for people who wish he weren't a crusty conservative", and unsurprisingly, since the originals weren't my cup of tea, I'm not really here for the remakes either, but perhaps other people have ideas about whether they'd be appropriate?