From Vampire Knights:
Rei and Seiko ran away from an orphanage, but had the good luck of being taken in by a generous rich grandfather. But because they’re not sure how long he has to live, they are in constant fear of being thrown back onto the streets again. And so they made a plan---to go to a prestigious school and pretend to be princesses born into a rich family in order to snag an elite who’ll set them up for life! At least, that was SUPPOSED to be the plan....