First off, good on you for getting a job along with school, and good luck!
Don't let your job get in the way of school. I guess I'm kind of biased, but I think I would say the same thing regardless. If you plan to go to college, how you do in school as a 16-18 year old will have a very significant impact on your life in the short term. If you don't plan to go to college it's less significant, but not zero. How well you do as a KFC employee will not have a significant impact on your life.
For most jobs, all you need to do to not get fired is 1) show up when you're supposed to, 2) don't be an asshole, and 3) don't be stupid. For a KFC job that will probably be enough to make you exceptional. I think it's good to get used to being diligent at your job, though. Often it takes one or two jobs before people really build the mindset they need to succeed at work. So imo, try to practice identifying what would make your boss(es) happy with you, and (if it's not too toxic or unreasonable), doing that.
Ultimately, the point of this is just to make some money, and maybe get a positive reference for a future job/college program/whatever. Don't lose sleep over it, just do your best and enjoy your discounted chicken.