The New Testament letters were written to real communities wrestling with real problems: theological confusion, moral failure, social division, persecution, doubt, and the challenge of living distinctively in a hostile culture. For secondary students in similar situations — navigating identity, peer pressure, scepticism, and the challenge of being publicly Christian — the letters address their actual lives with remarkable directness.
BibleProject has free overview videos for every New Testament letter (Galatians, Philippians, 1 Corinthians, James, etc.) — watch the relevant one before teaching from any letter to understand the historical situation and theological argument.