So you lead and the beginner follows, swimming behind you where you don't see him. Not good. Why not swim next to each other along the line? The diver in the back is actually solo diving. He may be in serious trouble but nobody notices. If you had swum for 20s longer before looking back, maybe you wouldn't have found him in time.
I think swimming in line is a frequent mistake. Plenty of accidents where the buddy says they somehow lost each other, actually the victim stopped behind with a problem and nobody noticed until too late.
Good point, and something I'd overlooked. Thanks for bringing it up.