Sinful Things by Elizabeth Briggs

They tried to intimidate me. They tried to get me expelled. They tried to bring me to my knees.
They failed.
The angelic princes betrayed me, and now it’s time for this little succubus to get her revenge.
My secret’s been exposed, but I’m not leaving Seraphim Academy until I rescue my brother. I know where he is, and I’ll do whatever it takes to bring him back—alive.
Even if it means going up against the secret society that uses murder and blackmail to control things from the shadows. They may be powerful and ancient, but I’ve infiltrated their ranks and I’m not afraid of them.
Especially with my sexy—and totally off-limits—professor helping me take the society down. I might even get the angelic princes to help me too, if they can behave this time.
But I’ll have to be careful, because now that the angels and demons know I exist? They both want to control me—or they want me dead.