Spellbinder by Lisa Jane Smith, now you can read online.
Expelled.
It was one of the scariest words a high school senior could think of, and it kept ringing in Thea Harman's mind as her grandmother's car approached the school building.
"This," Grandma Harman said from the front passenger seat, "is your last chance. You do realize that, don't you?"
As the driver pulled the car to the curb, she went on. "I don't know why you got thrown out of the last school, and I don't want to know. But if there's one whiff of trouble at this school, I'm going to give up and send both of you to your Aunt Ursula's. And you don't want that, now, do you?"
Thea shook her head vigorously.
Aunt Ursula's house was nicknamed the Convent, a gray fortress on a deserted mountaintop. Stone walls everywhere, an atmosphere of gloom-and Aunt Ursula watching every move with thin lips. Thea would rather die than go there.
In the backseat next to her, Thea's cousin Blaise was shaking her head, too-but Thea knew better than to hope she was listening.
Thea herself could hardly concentrate. She felt dizzy and very untogether, as if half of her were still back in New Hampshire, in the last principal's office. She kept seeing the look on his face that meant she and Blaise were about to be expelled-again.
But this time had been the worst. She'd never forget the way the police car outside kept flashing red and blue through the windows, or the way the smoke kept rising from the charred remains of the music wing, or the way Randy Marik cried as the police led him off to jail.
Or the way Blaise kept smiling. Triumphantly, as if it had all been a game.
Thea glanced sideways at her cousin.
Blaise looked beautiful and deadly, which wasn't her fault. She always looked that way; it was part of having smoldering gray eyes and hair like stopped smoke. She was as different from Thea's soft blondness as night from day and it was her beauty which kept getting them in trouble, but Thea couldn't help loving her.
After all, they'd been raised as sisters. And the sister bond was the strongest bond there was ... to a witch.
But we can't get expelled again. We can't. And I know you're thinking right now that you can do it all over again
and good old Thea will stick with you-but this time you 're wrong. This time I've got to stop you.