Here's a summary:
* Plot: The protagonist, Claire, is a shy and unpopular girl who desperately wants to be liked. She makes a wish at the annual school carnival, and to her surprise, it comes true. Suddenly, everyone becomes incredibly friendly to her and she's surrounded by popularity. However, there's a catch: the popularity is only temporary and will vanish once they graduate.
Let me know if this is the book you were thinking of!