You can bend over backwards to keep the peace to a point. After that point, you're just being bent over. I'm guessing you're a year or 2 older, just stop her in her tracks and tell her to quit. Quietly. When she says, "NO I'll.......blah blah", you just say, "I said I'd do it, go find something else to do.". She's taken charge like a spoiled pony, and now when you tell her no, be ready for a bit of resistance just like the pony would give. Stay calm, cool, fair and firm, she'll let it go or she won't. You can get the instructor involved at any point.