I got this novel from a Scholastic book fair at my elementary school in the mid 90s. It featured a kid who regularly played MUDs with his friends. In the text-based game world, he had an unusual power to discover secret keywords to perform super-powered actions. The plot went back-and-forth between his game sessions and his real life normal kid problems. By the end of the story, he gained real life powers mirroring those of his character in the game.
I was really young and can't remember any character or location names, but I'd love to find this novel again.
EDIT: Added more detail about the game world.