Wait until there's no time constraint (like class) to approach her. This makes it easier. Go up to her and say "Hey, I'm stuck with this douchebag (say his name if you're both familiar with the guy) for this project in studio lighting. I'd really like it if we could partner up for the next one." She'll appreciate no beating around the bush and the straight forwardness of it.
One thing I can tell you confidently is that it doesn't really matter what you say, as long as you have confidence when you say it.
