OK, I'm drawing a blank.
Which episode had Nog on a starship with a bunch of academy trainees/cadets that ended up behind enemy lines and attempting a risky action against the other side under the leadership of one of the cadets. All of the crew except Nog and a female cadet end up dead.
"Valiant", seventh (last) season. And the U.S.S. Valiant foolishly attacked a Dominion batttleship, which then destroyed them with ease. Besides the aforementioned female cadet, Jake and Nog were the only survivors.
Oops, I should have said "sixth season".
Thanks, Andre.