When it comes to the "general public", I find that being lighthearted but clear generally gets the message across. If you tell them what she needs to do and they say "oh, I don't mind if she jumps!" say something about her not knowing who is OK and who isn't .. I've said stuff like "You don't mind, but my grandmother does and my pup doesn't know the difference". The more consistency you get, the quicker it'll sink in.
You can also try shortening up the leash so that if she does jump, she can't "land" on them. It doesn't solve the problem, but she doesn't get the reward of reaching them so it makes the misbehavior less fun.
Maturity will also help.. the more meeting people becomes "old hat" and sitting get rewarded, the more likely she'll offer the behavior. When you get her in class, if they cover sitting nicely for a greeting, she'll get lots of good reps in with the trainer (and hopefully others).
