The Poison and the Cure by Corissa Blakely

Hillary Sterling must pay for selfishly and remorselessly breaking up with my brother. To get revenge, I’m enlisting the help of our high school’s resident mysterious bad boy to seduce her and break her heart. She’ll never see it coming.
I’m a nobody at school—the weird girl who wears plaid shirts and screws up grading curves. But in conspiring with Stephen Harva, I invite trouble into my previously uneventful world. He’s the most selfish and malicious person I’ve ever met, and he is determined to bring me down to his level and unlock my secret desires. He will lie, cheat, and even resort to blackmail to get what he wants. It’s my fault. I made the choice to play with fire, and I got burned. What started as an elaborate revenge fantasy is quickly spiraling out of control, but Stephen is a human hurricane. I can’t stop him.
And…I’m not totally sure I want to.