Hi cdc,
Whether you're planning to apply to Business School in 2019 or 2020, now would actually be a pretty good time to reach out to an Admissions Expert. Since it's a relatively 'slow' time of year, most of those Experts should not be too busy (so you probably would not need to wait for a response).
When I suggested that you might want to take (or retake) a College-level class, I was not suggesting that you had to retake a class at the same School as before. Thus, you might potentially take a class online or at a different (re: local) School. As long as the subject matter is appropriate and the School/class is reputable, then earning a high grade in such a class would still be beneficial to your applications.
GMAT assassins aren't born, they're made,
Rich