Por este post do Reddit , parece que a história se chama " Timor and the Furnace Troll ", do Livro de Monstros de Bruce Coville . Sua entrada ISFDB é aqui , e também foi resumida em esta postagem intitulada "História mais doentia que já li" (grifo meu):
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.