Students already take their keys everywhere, let them take their prom memories everywhere too. You can add a class photo to the key chain or any other meaningful photo and let them carry their memories with them always.
The great thing about giving away keychains as favors is that they're pretty timeless. People have been carrying keys for centuries and they're not likely to stop soon. Even in modern systems where you scan in with a card, many people still opt to keep their cards on a lanyard or key chain. It's a safe bet that students will love their personalized keychains.