In a podcast/interview, I heard Tim Jacobus say that he had created a cover for The Incredible Shrinking Fifth Grader, and he never released it. This really peaked my interest, so I figured I would contact him and ask him a few questions. To my luck, Mr. Jacobus sent me this:
After talking with Jacobus, I theorized that R.L. Stine actually did publish the book The Incredible Shrinking Fifth Grader as a one-off book in 2001. The book was titled "The Adventures of Shrinkman" - and the intro to the book says it was partially inspired by the movie "The Incredible Shrinking Man". I am uncertain if the stories are the same, but it is very, very, likely that Stine re-used the outline for The Incredible Shrinking Fifth Grader for the book The Adventures of Shrinkman.