I think this would have been in one of my textbooks around 1990 or so, 4th to 5th grade material in Kentucky.
A boy gains access to a time machine of the "visit your future self's body" sort. I believe either a parent built it or a scientist met on a field trip. He uses it at least twice to explore career paths. One was as a football player and one as a mechanic. In both, he was dissatisfied. As a player, he was tired, and aching from injuries. As a mechanic, he realized he'd never finished school and was stuck in a dead end job that didn't earn much money.
Finally he talks to the scientist and, I think, comes to the conclusion that the future is not set, that he can be whatever wants, but that he needed to stay in school to maximize his opportunities.
It was a short story, probably under a dozen pages, maybe as few as 2-3.