Wow, she's a tough one. I thought Di's suggestion was great, but your friend is one of the more clueless ones. At this point you need to make it obvious which of you is the problem because right now you both are if other women are getting annoyed. Frankly, she keeps talking because you keep breaking down and talking back to her.

Subtlety hasn't worked, so it's time to be direct. Don't ask her, tell her: "Talking during the class isn't fun for me, so I'm going to be concentrating more on the instructor. We'll have to save our chatting for afterward." Then follow through: Don't talk to her at all during exercise no matter what she says except to kind of point at the instructor and signal that your attention is there.

Your absolute last-ditch effort is to tell her to leave you alone and walk away, but I hope it doesn't have to come to that. Good luck!