Is your reason for being there totally lame? No it is not. Now stop thinking that and get your confidence back.
Well...you're intent was not too write a book. Don't feel compelled to do something you didn't intend to do. If you did write a book, that's great, if you don't, don't stress yourself out about it. Writing a book is beyond the scope of your project. Stick to your original goal.
Regarding what to say -- no matter what advice anyone offers, it still comes back to you. It’s your reasons, your goal, and your feelings. It has to be genuine and it has to come from you.
With that said.
First, don't think of it so much as an interview, or at least as an "official", "by the book", "do as you learned in school" type of interview. Basically your interested in the experiences these veterans had while in the military. However, I personally don't know if I would refer to their experiences as "war stories". I'm not sure if that possibly could be taken the wrong way by someone.
Second, like I mentioned before, don't feel compelled to write a book. Writing books and papers are a traditional method to store and pass on information. However as a teacher the things you teach your students are just as important. The knowledge, values and stories you share can make a lasting impression. During the time you spend with these veterans, you're bound to gain something. This may be a deeper appreciation of what they lived through, their personal struggles, stories, or whatever else. Whatever it is, it's bound to have some affect on your life and possibly even the lives of those around you.
I'm not sure if that makes any sense. I guess to put it simply. I had teachers who felt very strongly about certain things. As a student, I became very attuned to those feelings, either from their instructions or admonishments if I crossed the line. There are things my former teachers have taught me and I remember to this day. I can honestly say that my teachers have influenced opinions I have on certain subjects.
Next time (hopefully there is a next time), take in a model or two to show them so they understand -- it might even be a good icebreaker. Tell them that in your free time (which you have more of now because of summer) you like to build models. While you're building, you like to learn more about what it was like for the soldiers who served during that time, their experiences, feelings, etc. Maybe ask a few questions as an example. What were some of the places you were sent to while in service? Were you expecting to be sent overseas? Did you initially expect the war to last long?
Start out slow -- take small steps. When they feel comfortable talking I'm sure you won't need a list of questions.