The Popularity Papers: Book Seven: The Less-Than-Hidden Secrets and Final Revelations of Lydia Goldblatt and Julie Graham-Chang by Amy Ignatow
Lydia and Julie have been through many adventures as they navigate junior high, popularity, families and friendship. In the final adventure in the series, the peaceful world of Hamlin Junior High is rocked when Lydia and Julie learn that they're going to have to play host to new students whose junior high flooded. The outside threat bands the Hamlin kids together against a common enemy -- for a while. When the enemy gets their hands on Lydia and Julie's notebook, no one wants anything to do with them. It's the biggest threat to their friendship (and a pretty definitive failure of their quest for popularity) that can only be solved one way. Yes, it's what you're thinking: DANCE BATTLE.