A: In the "in crowd."For example, I was a solid C, initially a nerd, later a weirdo. Sarah was a B, generally well liked, class president, and friends with the A kids, but also friends with C kids and a little too brainy to be in-in.*
B: Not in the "in crowd" but socially in the mix.
C: Nerd or weirdo.
D: Viciously ostracized outcast.
* Sarah quibbles with my analysis, stating that while she was, in fact, a B in middle school, she was an A in high school. My response is that her high school was a magnet school for the arts and heavily skewed towards "nerds and weirdos." An arts-school A generally translates to a B in any "normal" school.