I read this story when I was in 7th/8th grade in ~1999. It was on a list of recommended "books" to read for a reading contest at my school. I don't remember if it was part of a compilation of short stories, or a longer novel.
The main character is a young boy, maybe 12 or 13 years old. It was set in a dystopian future Earth where some tyrannical force (I want to say aliens but honestly don't remember) finds humans and modifies their bodies to become monstrous. The one example I remember was a pterodactyl-like creature found by the boy; its skin was stretched taut and its eyes were tearing up with pain, and it begged the boy to kill it.
The boy belonged to a hidden group of children and young adults who were guided and basically commanded by an AI machine designed to help humanity to recover. I remember there being details about a lottery system for mating, and how much the boy hoped he was selected to be with a girl he liked.
At some point, the boy was out of their hiding place and upon return found the hidden kids being bound and led away, and the AI machine being fitted into a new chassis. I think he confronted the machine and it explained that it will be much more capable saving humanity with the new hardware.
I occasionally remember this story because of one of the first two points: a body horror begging for release, and barely pubescent children being directed to have sex; and how crazy it was that I was recommended this book to read when I was only 13 or so. I hope someone recognizes it because it's been on and off my mind for a long time. My own searches never showed promise because of poor Google-fu and being unable to combine these details together, each of them alone showing many results that didn't fit.