There are no guarantees in life so, yes, you might be rejected. If she is as wonderful a person as you believe she is, then she'll be kind about it.
But let's not talk in negatives.
You go up to her and you say "Hi, it's been a few years. How have you been? What have you been doing with yourself?"
If she's in a group, you say hi to everyone, you stick out your hand and say, "I'm Randomguy354 (or whatever your real name is). Susan (or whatever her real name is) and I went to high school together."
You have your script now. Do you know 100% of what is going to happen? I'll let you in on a dirty little secret.
No one does.
And the not knowing doesn't have to be scary. Consider it an adventure.