There's Magic in the Air

Emma was an ordinary, 10 year old girl. Every year on her birthday, she would go to her Gran's house to play under the stars, with fireflies all around. But when her Gran moves to Florida, things change a bit.

7 years later, Emma turns 17, and a mysterious boy asks to talk to her. He tells her she has magical powers, and that her Gran has been missing for 7 years, and it's up to her to get her back

(Just so there isn't any confusion, the story is happening when Emma is 17 years old)