Hex Hall by Hawkins, Rachel Hawkins

Three years ago, Sophie Mercer discovered that she was a witch. It's gotten her into a few scrapes. Her non-gifted mother has been as supportive as possible, consulting Sophie's estranged father???an elusive European warlock???only when necessary. But when Sophie attracts too much human attention for a prom-night spell gone horribly wrong, it's her dad who decides her punishment: exile to Hex Hall, an isolated reform school for wayward Prodigium, a.k.a. witches, faeries, and shapeshifters.