The memory of this story has stayed with me since my childhood, and I'd love to track it down someday.
I would have read it in English, in the USA, in the 1980's. I have the impression that the story or book was new at the time, but I can't back that up with anything other than a vague gut feeling.
It was in a collection of short stories, I would have gotten it either in a library or from a scholastic book catalog - I think it's more likely a scholastic book, but, I don't recall.
The basic plot of the story is that a group of young boy-scout aged boys were going camping with a new group leader - a man they had never seen or met before.
Something seemed 'off' about the man, but, as he was the assigned chaperone, they all went along with him.
While on the camping trip, one of the boys disappeared, so the camp leader led the rest of the troop off into the woods one at a time, using the excuse of setting them up in a grid pattern (or something similar) to help find the lost boy.
The last two remaining boys sensed that something was very wrong, so the second-to-last boy promised to whistle or call out or some sort of signal once he was away from camp and in place.
The remaining boy waited while the chaperone/troop leader guy led the second-to-last boy off into the woods, waiting for the signal. All he heard was silence, except for what might have been some sort of strangled gasp, quickly hushed.
The last remaining boy (our doomed protagonist) then noticed that the moon was full, and the implication was that the camp leader guy was actually a werewolf, and that he'd now be coming back to camp to devour the last boy.
I don't think that the werewolf aspect was ever explicitly described for certain, and I don't think that the last boy actually died at the end - although it was strongly implied that he was doomed in the extremely near future.
This was almost certainly a YA book - there was no explicit gore or even explicit deaths that I can recall. We don't know for sure that the missing campers have been killed, and we don't quite know for certain that the camp leader guy is a werewolf. Just lots of hints and clues.
I remember really liking the story - the memory of it has stuck with me for over 20 years, I'd love to find it and read it again.