Complex sentence: Bonner never received less than an "A" as final grade in any class throughout his academic career, until he received a "C" on a chemistry exam in college, which led to a final grade of a "B".
Split into simpler sentences: Bonner never received less than an "A" as final grade in any class throughout his academic career, until he received a "C" on a chemistry exam in college. Which led to a final grade of a "B".