Well, we know Nobara is great at martial arts. It could translate somewhat, and if she leads maybe Yukino will forget her nervousness and just follow and end up dancing decently. That's how it usually happens when these things are boy/girl.
Top or Bottom. At least with fixed partners you don't get confused how to step. So this is a +. Having a partner chosen for you well this is the - hurdle, if you not have made a friend on the other side and just have learned a year long to hate the others.