I would possibly postpone the meeting for a while and continue talking to them online to see if you're still friends. One or both of you might have changed and you don't really get on anymore, in which case it would be pointless meeting up and coming out, especially if you don't know if she's the sort of person who would tell others now.
If it does work out that you still both want to meet up then I would tell her before I saw her, what you say would depend on whether or not people knew why you left school, I'm assuming they don't. I would say something like, "you know I left school? I was going through a lot of stuff, and as you might be able to tell, I'm living as a guy now." Don't make it out to be a big deal.