as a black and white photograohy major, the school outfit would look the best in the pictures because it has more contrast.
The rose meet dress would also look nice, more subtle but still contrasty. I don't think the purple dress would look good and the summer dress, too white, the christmas dress, too dark., her birthday dress could look nice too.
I can't wait to see a picture of this. Are you gonna be able to show us when you're done?