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I think the author did this to show Melinda's state of mind despite her success in gym class. Melinda's depression continues to affect her ability to fit into her world, even when it does make places for her. She has her first success in gym class, but is unable to appreciate it, and isn't interested in joining the team, even if she could get her grades up. She does respond to her teacher's request to have her teach the basketball team how to throw foul shots. She is not happy to be kept after class to show off her basketball talent. Melinda would rather be alone, and not the center of attention, and she is not ready to join any groups, as much as she might often dislike her isolation.
