Witch Island by David Bernstein

A witch's curse from beyond the grave!
Witch Island used to be feared. Even the bravest would not dare go there. Legend said a witch had been burned alive at the stake, and upon her death she cursed the town. Terrified residents performed rituals to keep her spirit trapped on the island where she was buried.
Now, over a hundred years later, a group of high school seniors have decided to forgo the local graduation parties and have a small gathering of their own-on Witch Island. They don't fear the legends. They scoff at them. But the group will soon learn these particular legends are nothing to scoff at. And Witch Island will prove far worse than they could have ever imagined.