Hello, you will find lotsa information about the movie here
The secret is under the spoiler tag, but I highly recommend that you watch the movie without knowing the secret. The movie is certainly worth watching.
click to show/hide spoilersThe girl was somebody from the past, she traveled to Jay's time by playing a piano piece using the old piano in the old school building. Every time she wants to go back, she had to play the piano piece very fast. Whenever she time travels, only the first person that she sees will be able to see her, she will be invisible to everyone else.
That's basically the secret, of course there's more to the storyline than just that. The first half of the movie is rather heavy on the cheesy high-school love story scenes, but the movie really picked up after that.