First, congrats on the offer. Second, visit the class. It’ll be great experience for you. When you do have a published book, you can do another visit with that teacher’s class—which will be full of entirely different students anyway. No need to turn down this opportunity because the book isn’t pubbed yet. You can expose this group to the writing/publishing process and to your work . . . and it would sure be inspirational to them. Bonus: You would get in some presentation practice when the stakes don’t feel so high. You’ll have plenty of events in the future for which you’ll worry about having books on hand; for now, get your speaking feet wet in a friendly setting.