smr, I don’t think you’re right about BJJ having a very yeshivish parent body. I get the impression that BJJ (and Bnos Sarah) have a much more diverse crowd than Hadar, and lots of the students don’t come from typical yeshivish backgrounds, as opposed to Hadar.
BJJ likes to accept girls from interesting backgrounds and places. They accept a percentage from all schools. So if someone applies from a very out of town place or from a more modern high school, they have a better chance of getting accepted than someone who applies from in town. From my grade in Lakewood, very few girls went to BJJ and lots of girls went to Hadar. This was a while ago, but it doesn’t sound like much has changed.