Who is God? Who am I? Who Are You? by Dee Pennock: This book covers many difficult topics for teens about living in the world. Some of us have used it in place of Charlotte Mason's Ourselves for citizenship. It has so many good quotes and comments free will and how to love our neighbor, all in the light of the Orthodox phronema. Encourage your students to get past the funny illustrations and really dig into this excellent text. Unfortunately, this book often goes out of print and can be hard to find. Check with your church school director to see if they may have copies of it you could borrow.
Among the Saints graphic novels (three books): Excellent graphic novels full of drama and suspense. These would be a great introduction for your Form III students if they have not yet read lives of the saints independently.
Letters to Saint Lydia by Melinda Johnson: It's a little difficult to categorize this one, but it's about a young woman whose family converts to Orthodoxy, leaving her with many questions. As she faces all the temptations of teenage life, she "pours out her troubles in letters to St. Lydia, who (invisibly to Lydia) answers, guiding her through her time of troubles with deep love and compassion" (from the publisher's website.)