Once you graduate and are accepted at/attend a college your HS years will be ancient history. That college C in my major in college is also ancient history. So are medical school P/F grades and my board certification was for life (before they went to 10 year ones). I got my MD, and passed licensure exams. I would guess the only use of HS or college grades is that eventually your own kids can point out that slipped up when they also get a bad grade. I have discarded my old school grade reports- they can’t be held against me by my son (oh, they were excellent in general).
The importance of HS grades is showing you learned the material and know how to study in college. They count for college admissions. As above- “the next phase of your life” and what you do with it is what matters. Each step overrides the previous one. Nobody cares how you did in middle school, nor elementary either.