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Kids can be cruel, whatever country they live in, and they like to gravitate towards people who are similar to themselves. There are groups of more popular kids and, more sporty kids, and more arty kids wherever you go. Non-Americans seem to see the problem as being particularly American, however.
I think this stems from movies again, where we hear a lot about the nerds and the jocks, and the cool kids. It may not be any worse over here than anywhere else, but movies have focused on it so much that sometimes new students are afraid that they won’t fit into any group.