Hey there, graceomalley. Let me say, first that I am not an English teacher or even a parent; however I do have fond memories of good novels from those years you mention. In ninth grade, I believe it was, I read the classic J.D. Salinger novel "Catcher in the Rye." I remember "Football Dreams" being a good one --- I liked the novel even though I don't care for football and never have (I don't remember the author). In the eleventh and twelfth grades you might challenge them with Steinbeck's "Grapes of Wrath."
I think a ninth grader can handle Robert Louis Stevenson's "Dr. Jekyll and Mr. Hyde." At that level I remember we read "Things Fall Apart" by Chinua Achebe, the Nigerian writer. You know the drill.
Here's a few unorthodox proposals I hope you might consider: "The Dain Curse" by Dashiell Hammet (I think they can handle that in ninth grade); how about Tarzan novels by Edgar Rice Burroughs for seventh and eight grade [you laugh but those books are quite good in spite of your memories of the cartoon and old-time Sunday Tarzan show]; how about a novel called "The Portrait" by Ian Pears (the book is quite short for a novel, but nevertheless this should only be given to the eleventh and twelfth graders, I think.
I could be wrong but I think anything by Patricia Highsmith should only be given to a twelfth grader. I think 9-12 might enjoy the crime novels of Elmore Leonard (maybe save John Grisham for 11 and 12).
And don't forget to feature a generous helping of the science fiction novels of Phillip K. Dick. And don't forget about the fantasy writer Harlan Ellison (for grade 12 only, I think).
Great question. Thank you.