Sure, finding that sweet spot for a teen who reads at a high school level but might not want super adult themes can be tricky. 'The Book Thief' by Markus Zusak comes to mind, it's right around that 850-900 range. The subject matter is heavy, obviously, but it's told through Death's perspective and the prose is deceptively simple yet deeply poetic. It handles mature themes in a way that feels accessible and profound, not gratuitous. Another solid pick is Lois Lowry's 'The Giver'. That one has haunted me since I was a kid, and it's a cornerstone of dystopian fiction that makes you think without being overly complex.
For something a bit more genre-focused, 'Ender's Game' by Orson Scott Card fits. It's a sci-fi classic with strategic depth and ethical questions wrapped in a plot about child soldiers, which sounds dark but is utterly absorbing. The Lexile is high, but the narrative pulls you along so fast you don't get bogged down. I'd also toss in 'The Curious Incident of the Dog in the Night-Time' by Mark Haddon. The writing style is unique, first-person from an autistic teen, and it tackles a mystery with an emotional core that's surprisingly moving. It feels real and raw, not like it's written down to anyone.