Identify a story about elf and troll and cardboard box

Question

Please help me identify/remember the name of this short story (and/or the name of the anthology book it was in). I remember reading it in the mid-90's cannot remember the name of it or the book at all. The plot goes like this:

An elf (I think, or some other harmless creature) wants to do a school project on trolls (or some other evil creature). He is told to use a cardboard box which has the power to stop time so that he can study for a long period and finish the report quickly (relevant to everyone else who is stopped). To get it he should go talk to the troll who runs the boiler room at the school. He does so and learns that trolls have been treated badly and starts to befriend the troll in the boiler room. He then learns that there is a way for him to become a troll.

All he has to do, is eat the other elves in his class. Which we does with the help from the troll in the boiler room.

Answer 1

Per this Reddit post, it looks like the story is called "Timor and the Furnace Troll", from Bruce Coville's Book of Monsters. Its ISFDB entry is here, and it's also been summarised in this post entitled "Sickest story I have ever read" (emphasis mine):

The story itself is about a child elf named Timor. Unlike the other elves, Timor stinks at magic and is the target of much teasing in his school. [...] In the backstory, there was apparently a huge war between elves and trolls, with the former winning.

During a school assignment, Timor is sent to the school furnace room to gather information for his assignment I guess. There he meets an old troll named Mr. Alfmordorson. Pretty much, Timor and Mr. Alfmordorson forge a friendship. I remember there was something magic about cardboard boxes and such and well I distinctly remember they held a certain kind of bond. It goes to the point that Timor wishes he was a troll. [...] Timor then teleports back to his class only this time, he brought the troll with him.

It's here we learn what trolls eat. Warning, what you may read now will probably sicken you. Mr. Alfmordorson snatches Timor's teach, stretches her spine out and eats her. Timor then lowers a giant fork to his frightened class and forces them towards Mr. Alfmordorson who proceeds to eat the elf children, one even screaming as she is eaten alive. It gets even darker as Timor takes the giant fork and skewers one of the kids who tries to escape. The only one Timor doesn't feed to the troll is his "friend" Greta. Now if you are imagining a happy end, turn around. Through narration, we learn that Timor eats Greta, wiping his mouth with a hank of her hair. The story ends with Timor transforming into a troll.