Changing the Rules by Catherine Bybee

As an employee of MacBain Security and Solutions, Claire Kelly can certainly hold her own. Armed with an impressive set of covert skills, she’s more than prepared to tackle any job that comes her way…except one involving Cooper Lockman.
Cooper and Claire used to work together before his feelings for her sent him packing to Europe for six long years. But now he’s back and determined to ignore the still-smoldering heat that lingers between them.
Their current mission: go undercover together at a California high school to root out the mastermind behind a prostitution ring targeting young girls. The closer they get to the truth and the closer they get to each other, however, the deadlier their task becomes. As Claire and Cooper risk their lives to bring down their target, will their hearts be the final casualties?