This is a tricky situation really. What you do is all down to your own judgement. Do you feel that at this point it would be right to bring this up with her? Let's consider the facts. All those years ago when she heard that you liked her she turned away. This could say one of two things, that it scared her and she didn't know how to react so she ran or that she doesn't really like you back and wanted to deter you, in hopes that your feelings would eventually die out.
Are you speaking to her now? Are you on friend terms? If so are you willing to jeopardise your current relationship by revealing all to this girl? This is all for you to decide. You are right, she could again run further and your chances with her will be completely be lost or she could have matured over the years and grown a little feeling for you back. you never know, it's all about whether you feel brave enough to give it a go.
But and it's a big but, you say that this girl is going with another guy to the prom, this is bad news for you. You need to ask around and find out whether they are in fact an item. If it turns out that they are then you need to back off for a bit. It is not fair to put these feelings onto someone who is currently in a relationship. It would mess her up completely and possibly cause a lot of trouble.
One thing i can commend you for is waiting until the GCSE season is over. I remember at the time of taking mine i had a lot of stuff going on with guy who completely messed me about and it only added to the stresses of exams and quite frankly really lowered my performance thus lowering my grades to a disappointing level. You are very wise not to do this to her.
You know, i really feel for you. I know what it is like to have feelings for someone for so long and be afraid of telling them the truth. I never did tell the person i liked and a part of me regrets it only because i don't know what could have been. I would be extremely careful with this, with your own feelings. When you idolise someone for a long period of time, it can be a huge amount of pain when the realisation that you two will never be sets in. I would suggest that you really think about this carefully and do whatever you think is the best for you both.
I wish you the best of luck and hope that you are able to make yourself happy. If you need to message me then please do.
-Cara