Plantinga is pretty dense and presumes familiarity with a lot of philosophy, even in the "lay level" books like Where the Conflict Really Lies. And if you haven't read the stuff he's in conversation with (Dawkins, Dennett, etc.), a lot of it is going to be hard to follow.
I spent some time culling through Well Trained Mind Threads and found these two resources that seemed promising for high school.
This text was recommended by several women as an epistemology text: Cambridge's Theory of Knowledge. This text Theory of Knowledge is setup to be used as an IB course. I can't quite figure out if it is a stand alone text, or if it goes with the Cambridge text.
This text was recommended by a professor of philosophy and religious studies as one he uses with college freshmen and planned to use with his own highschooler. Philosophy: The Power of Ideas
For both of these, you would have to add in your "Christian perspective." I would think of the texts as a foundation so your student could understand what Christian authors are referring to when they discuss philosophy and what the issues even are.