Elementary school messed with me long before I made it to high school. I was an unusual kid who spent much of my time reading and daydreaming. There weren't any other children like me in elementary school because the student body, although it spanned several grades, was very small. And kids of a certain age tend to pick on things they don't understand. Relentlessly. By the time I got to high school, the feeling of hatred was mutual.
I'd say there were more factors at play besides my introversion, though. I just had different interests and was accelerated a few grades in certain classes, which marked me as an oddball. Many people, and kids especially, dislike or distance themselves from that which they don't understand or aren't familiar with. Familiarity = comfort.
Time helps, a little. I like what Saturned says about the beauty of growing older!