This book has been out for a while now and it lends itself beautifully to both non-fiction Common Core English Language Arts standards and 8th grade CC Social Studies standards. In addition, there are excellent free lesson resources available for teaching this book.
My son read this in elementary school, far too soon to do a full exploration of it fully and the CC SS standards don't address WWII until 8th grade. While the Lexile level is on par with 4-6th grade reading, which means it's easily accessible to even struggling readers in middle school, students benefit from waiting until they're a bit older and have built some prior knowledge of WWII to tackle this, or are re-reading this after being exposed to it in the elementary grades.
It's also, quite frankly, a great book to teach because you don't have to buy all your class copies brand new, although many teachers prefer to do so. You can find lots of good, quality used copies for sale, and there are likewise lots of free teaching resources for this on my favorite pay-for-teacher resources vendor, Teachers Pay Teachers.
The book's publisher, Scholastic, offers free Discussion Guide questions and suggested answers.
Code Talkers: Montana English Language Arts Unit
By far the best free set of lesson plans for teaching this book comes from the state of Montana. This unit is 59 free pages of all kinds of great teaching materials and resources for an ELA approach to non-fiction standards with this book. I highly recommend you peruse it and decide in advance what you'll use. It's absolutely astounding!
Social Studies: Spy Kids: Code Talkers, Eisenhower Foundation Unit and Lesson Materials
I taught this unit in 8th grade Social Studies, to meet CC SS WWII standards. My students absolutely loved this! In advance, I prepared and laminated special "Top Secret" manila folders for table groups of 4-5 students. Each table was responsible for decoding their message and presenting the contents and contextual information to the class. Then students took notes on each other's presentations.
I can't rave enough about the Eisenhower Foundation's educational materials. My only gripe is I wish they'd offer even more!
I used the above jpeg, printed it out and laminated it onto the Top Secret File Folders, which contained each group's set of materials and secret mission envelopes. They were reusable and quite durable.
And of course, here is my editable lesson plan, adapted from the above unit and using all of its gorgeous materials.
I reiterate -- my students LOVED this lesson!