At the school I’m familiar with there isn’t a numerical ranking. It depends what the school needs, after the admitted students make their decisions about where to attend.
The school may find itself short of a day student 9th grade girl, so that’s who gets off the wait list, even though a 10th grade boarding boy has better grades and scores.
Maybe not enough violin players are coming, so that’s who gets off the list. The wait list helps round out the class.