Cliques.

Sometimes we stereotype and as soon as we see someone who gets straight A's is quiet, wears glasses, speaks silly, AS A "NERD". Or a cheerleader, who has plenty of friend, is stereotyped to be "POPULAR."
But lets think of how that persons life is in reality, if we would've really known that person would you guys be friends? Think about meeting that person before you judge. Sometimes other kids feel left out when there's cliques. Cliques most often bully when they get together. You could try to have as diverse group of friends as possible and don't get into the the clique scenario.
Don't judge a book by its cover.