(For Ivy League alumni/students) Did any of you get a B in highschool?

No, but I sure didn't take 3 AP cles as a sopre either. It sounds as though you're extremely driven, and as long as your subsequent grades are strong and you learn that "high school" is two words, you're well on your way to getting into a top school. A B/B+ will not single-handedly ruin your chances.