I am entering my senior year next year. I have somewhat impressive credentials (top 20 out of 700 in my class, 6 AP courses junior year, 7 AP courses senior year, plenty of extraciriculars and leadership positions. Most As and 5 Bs.)
But I got one C my sophomore year in one quarter (semester grade was still a B) will this take away from getting into a school like Northwestern?
I honestly don’t think so. I’ve personally never had a C when applying for colleges but I’ve seen plenty of kids with Cs get into the top schools. Plus as you said, your semester grade showed that you improved. I think you’ll be totally fine.
I don’t think one C will ruin your chances. Our school only reflects final grades on your transcript so no school would see a quarter grade anyway. You might want to check what will be shown by your school.