Hi BthB,
Not blowing my own trumpet, but are you using my guide to BGAE? One of the reasons I wrote the book was because there is so little that's available that is pitched at 17-18 yr olds. This should provide them with a clear understanding of the text, it's general arguments, some background, and some suggested avenues of critical evaluation.
There is also a list of secondary reading at the back. Here are the texts which I think would be most relevant to you:
For a broader perspective on Nietzsche's thought, "What Nietzsche Really Said" by Robert Solomon and Kathleen Higgins is excellent, and is perfectly pitched for your purposes. I shouldn't think many of your students will be interested in a detailed biography, but if they are, then R. J. Hollingdale's "Nietzsche: The Man and His Philosophy" is readable and interesting, and helps put everything in historical perspective. Finally, for something which has a small section on Nietzsche, but which deals with broader themes which Nietzsche would have a lot to say about, there is John Gray's "Straw Dogs", which is readable and provocative, and a good jumping off point for discussion.
Hope that helps!
G.