What grade are we talking about, 6 or 8?  That would make a difference, maturity-wise, of what to recommend.  

Not sure of the exact Lexile since they usually don't run Lexiles for adult novels, but my book club, which included barely-out-of-8th-grade 9th graders, read "The Night Circus," published as an adult book, a few years back and loved it.  Same story with "Ready Player One," also published as an adult title.  Maybe "The Martian?"  Take a look at ALA's Alex Awards lists, which all of these books have made.  The Alex Awards are for titles published for adults but having strong teen appeal.  That said, be careful choosing; they're thinking of more mature teens when they create these lists, so there could be graphic violence, sexual situations, moral ambiguity far beyond most YA novels, etc.  

Also consider YA or popular adult nonfiction, which generally has a higher Lexile because of the specialized vocabulary.  Of course, in that case you'd need to be sure that the students have the background--history, science, etc.--to make sense of the content.

All of that said, I'm often uncomfortable when a school asks students to read material that far above their grade levels.  Just because they have the decoding skills to read on a high school level does not necessarily mean that they are mature enough to understand high school/adult situations.  Having been forced to use the Accelerated Reader program years ago in an elementary school I worked in, I saw our genius 3rd grader attempting "Crime and Punishment."  Seriously?

Cathy Andronik

Retired, Brien McMahon High School

Norwalk