They're at the strangest in-between age. My daughter started reading 400-page Chrichton and Grisham then, and my son stuck to 100-page Wishbone. They're capable, but still love animals and silly things. They don't like condescension.
I think the Peretti Cooper kids series was perfect for that age group...exploring the unknown, out-of-the-box Christianity, adventure, not too many descriptive words to bog reading down.
Holes, Shiloh series, Brian's Winter and Hatchett, Ginger Pye also went over well then. I liked them, anyway!
You could place yourself into the life of some adolescents to hear how they talk and think---teach Sunday School for a bit, or take a neice to a soccer game. They're all distinct personalities, and you will never write one book that pleases all. Dive into it, and you'll find inner hunches about wording, along the way.