Based on what we have seen with our sponsor family over the years, USNA is used to seeing plebes grapple with Chem and Calc from a wide variety of academic backgrounds. Validation tests will help place plebes in the right course. There's plenty of EI, study groups in company, academic skills center and shared effort to help get everyone through. That said, a large number of plebes tank either Chem or Calc, or both, get seen by the academic review board, and have to take it over during the summer. Once you're at USNA and trying, the faculty and Commandant's staff do everything they can to help you succeed. As has been discussed elsewhere on this forum, time management skills, focus and minimizing procrastination are essential to being "sat" academically, physically and plebe-performance-wise.
Valdiation can be a wonderful thing to help jump ahead on the academic matrix, leaving time for other opportunities. But -- it's not so bad to not validate, and be able to manage a course more easily for perhaps a better grade than the "stretch" course. Anathema to over-achievers, yes, but sometimes optimization is a good strategy for plebe year.