I'm not sure I read all of these when I was actually 12, but they are a few of my favorites from late elementary school and early junior high.
- Anne of Green Gables and Anne of Avonlea by L. M. Montgomery
- A Wrinkle in Time, A Wind in the Door, A Swiftly Tilting Planet, and others by Madeleine L'Engle
- The Wizard of Earthsea, The Tombs of Atuan, The Farthest Shore, Tehanu, The Dispossessed, and others by Ursula Le Guin
- Heidi by Johanna Spyri (I used to read this every spring, from early elementary school to early high school)
- The Adventures of Tom Sawyer and others by Mark Twain
- Cheaper by the Dozen by Frank B. Gilbreth and Ernestine Gilbreth Carey
- Ender's Game by Orson Scott Card
- Jurassic Park and Andromeda Strain by Michael Crichton
- The Pearl by John Steinbeck (it terrified me, but I couldn't stop reading)
- Mythology by Edith Hamilton (I read this repeatedly and became fascinated with constellations and subsequently astronomy because of it)
- The Little Prince by Antoine de Saint-Exupéry
- The Westing Game by Ellen Raskin
- The Doomsday Book by Connie Willis
Throughout junior high I read all of the science fiction books that won both Hugo and Nebula awards, and enjoyed many of them, though most weren't that memorable. I also read the Valdemar Trilogy by Mercedes Lackey, which was also enjoyable but not particularly memorable. There are also many others I read that I don't remember. (Maybe you can remind me with your favorites. :~))