I used to have a 'thing' about people who weren't in the same grade as me. As in, somehow I got the impression that it was 'wrong' to make friends outside my grade. My mother certainly discouraged me from making friends with anyone that was at least a year or two older or younger than myself. Looking back it seems very strange.
Additionally, I found that when in school we had split classes (part of the kids in my grade in another room), and I had less contact with the kids in the other part, I sorta drifted further away from them.
I also had another wierd problem...I resented anyone who was older than I was by the order of a few months. Not that they knew this, it was a secret resentment. But if someone was more than a couple years older, it did not bother me.