The single biggest mistake in Sunday School planning is using one lesson format across every age group. What holds a five-year-old's attention (movement, repetition, simple crafts) will bore a twelve-year-old, and what engages a middle schooler (discussion, real-world application) will lose a preschooler entirely.
For early elementary, keep lessons short (10-15 minutes of teaching, maximum) and paired with a hands-on activity that reinforces the same single idea — one Bible story, one takeaway, repeated in a craft or game.
Upper elementary and middle schoolers respond well to a story-plus-discussion format: tell the passage, then ask open questions rather than yes/no ones — "why do you think he responded that way?" invites more than "was that a good choice?"
Teenagers, more than any other group, want relevance over repetition. Anchor lessons in real questions they're actually asking — friendship, identity, pressure — and let the Bible passage speak into that question, rather than teaching the passage first and hoping relevance follows.