It's possible you're thinking of one of these books by Lisi Harrison:
* The Clique series: This series follows the lives of a group of popular girls at a private school. There might be a specific event or chapter in one of these books that refers to "Bratfest," but it's not a standalone title.
* The Clique spin-offs: Harrison has written several spin-offs, including "The Clique: The Gossip Girl" and "The Clique: The Boy Next Door," which might have a similar concept to "Bratfest."
To help me find the book you're looking for, could you please provide more context?
* What are the main characters?
* What is the general plot?
* Where did you hear about this book?
With more information, I can help you find the correct book!