I believe you are referring to a Back-to-School promotion. This is something Apple typically does in Summer. Last year the deal included free Airpods. This year I believe it is a Gift Card (determined by the product you purchase). This could be different depending on which region you're in.. in the US details are available here: Back to School - Education Discounts - Education - Apple
You would need to return to the store you purchased the item or contact online at 1-800-MY-APPLE to see if anything can be done for you.