Conjure Island / Eden Royce.

Enhanced descriptions from Syndetics:

From the award-winning author of Root Magic comes the story of a girl forced to spend the summer with a great-grandmother she's never met--only to discover she runs a school for Southern conjure magic.

If you ask Delphinia Baker, she'd tell you she has all the family she needs. Sure, her mom passed away when she was young, her dad is often away on deployment, but even though Del has never had anyone she can call her people, she has always had her grandmother--and that's enough. Besides, having no roots just makes it that much easier when you have to move again.

All of that changes, though, when Gramma falls ill and Del is sent to stay with her great-grandmother. Del has never even heard of Nana Rose, and she has no interest in spending the summer on an unbearably hot island off the South Carolina coast. And when Nana Rose starts talking about the school she runs dedicated to their family's traditions--something called "conjure magic"--Del knows she's in for a weird, awkward summer.

That is, until the magic turns out to be real.

Soon, Del is surrounded by teachers who call themselves witches, kids with strange abilities, creatures and ghosts who can speak to her. She has a hundred questions, but one more than any other: Why didn't Gramma ever tell her about her family, the island, this magic? As Del sets out to find her place in a world she never knew existed, she also discovers a shadowy presence on the island--and comes to believe that it all might be connected.