As a novice lead student I have confronted this issue many times. There often comes a time in a lesson where it's just too advanced for me. When I get rotated with a new lady I just flat out tell her I will not do "such and such" move because I don't know it yet. Sure, they are disappointed, but most of them probably feel that it's better that I know my limits. Sometimes they will help me to learn the lead and that's OK as long as we slow down. A few show snobbish resentment but I just ignore those and lead them within my abilities. Bottom line is that I'm the lead and so I decide what we are going to do.