My SIL did horrible in high school. Like all C’s. She got a full scholarship to a state flagship for a sport. Graduated and decided to go to nursing school. Then got her master’s and most recently her PhD. My husband and I were both solid B/B+ students in HS. Both went to good, not great, colleges but we have done very well for ourselves without any advance degrees.
So no, high school grades don’t define you forever. Although CC might make you think otherwise. ?