Since you are going to different schools next year, NOW is the time to make your move if you’re going to do it at all. It’s better to express your admiration for her now than regret not doing so. Yes, you risk rejection, but there’s a risk to everything. The real risk is never saying anything.
You don’t have to express undying love for your friend, just a special liking. Consider taking one of the following approaches -- whatever works for you:
1) Wait for a time when you are alone with her, then share how you feel about going to different schools. Describe how special she is and how much you will miss her. Based on her response, use it as a segue to say that you have to admit you have a crush on her.
2) Send her a photo slideshow of the two of you with a special “crush” background song. Being such good friends, I’m sure you have many photos that feature the two of you. Let the last slide be your big ask. Ask her out.
3) Send her flowers, and on the card that comes with the flowers say something like, “You’re beautiful. Will you go out with me?”
Whatever your choice, I wish you the best in sharing your feelings with her. She’s probably waiting to hear it.