This is a really good question and something to think about.
Peer pressure can be such a horrible thing on both sides of the equation....to the one bullied and even to the bullies themselves. As we are growing, we all make mistakes, none of us is perfect. You have learned from your mistake and that's a wonderful thing!
Let's see what you have to loose by apologizing to this girl. Hmmm...I can't think of anything!
Now, what can you gain by apologizing for your actions? Lot's of good stuff....
1. Taking responsibility for your actions
2. Admitting you made a mistake and trying to rectify it
3. Showing you truly do have compassion for others
4. Helping to relieve the guilt you feel by being mature and responsible
5. Possibility of making a new friend
Whether or not this girl truly ever heard you really doesn't matter. YOU know what you did and you now know it was wrong for whatever reason you did it. By apologizing to her you could start off by saying - "I don't know if you ever heard my words about you in the 8th grade, but I wasn't very nice to you and I would like to sincerely apologize for my behavior". I think that is a most generous way to speak your apology and it could truly open the door to some great communication and possibly a new friend

You'll sleep better at night too
Of course it is always possible that she did hear you and she's not prepared to accept your apology. That's ok, it's up to her. At least you apologized and you won't have to have this incident eating at you for years to come.
Wishing you well!

sabby