Why are you scared? What is the worst that can happen? The worst that can happen is that she is awful to you and you never speak again. Whilst that might upset you a little to start with, you will come to realise that she was not worthy of your friendship, but heck at least you tried. So that's the worst that can happen.
Do nothing - you will unlikely never hear from her or speak to her again. You might regret not having tried.
Text her/message her - wish her the best of luck in her new school but say you would like to keep in touch with her and perhaps meet up for coffee or something. You may never hear from her or speak to her again, but at least you tried. On the other hand, she may message back and say "hi, thanks. Yeah I'd love to keep in touch. I'll be back in town on (insert date) and perhaps we could do coffee then?"
And also seek some advice to work on your confidence in social situations. You have all the tools, it is just a case of using them in the right order.
You cannot change someone else's feelings.