Offer the 'victims' small prizes if they can swipe the rescuers mask during " panicked diver on the surface".
Wait until everybody thinks they've finished and spring a snap scenario on them. " Well done everybody! Hold on! is that a diver in distress over there?"
( we used to get the DM to climb onto the boat, cross over to the other side and jump back in. Frequently, nobody noticed until the skipper said with a smug grin, " Are you SURE that everybody is back on board?")
A friend of mine used to take a small pony bottle with him to the bottom, lie face down and undo all the clips on his BCD. Rescuers turn up, inflate victims BC which promptly polarises to the roof. Victim grins at rescuers and begins breathing off the pony.

I always think that the rescue course is more about creating the mindset than getting the skills down pat.