Why was befriending Claudia Regan in the sixth grade not a good move for Kelly?

Fifth grade had gone well but it was followed by a sixth grade that did not go well at all. Kelly and her friend, Allison Mundth, did everything together, from entering drawing contests to creating their own exclusive clubs. Kelly started middle school full of optimism. Kelly and Allison became friends with a girl named Claudia Regan. Claudia began being more friendly with Allison and basically taking her away from Kelly.