Since Bnoy Yaakov is only a high school it needs to attract girls from other schools i.e to feed them girls, hence the term feeder school.
Bnos Yisroel is a school in Flatbush run by Rebbitzen Groner (wife of the mashgiach of Chaim Berlin who passed away a few years ago). Bnos Yaakov was started by a former teacher at Bnos Yisroel. I beleive there are around 10-15 girls from this year’s 8th grade at Bnos Yisroel who will be going to Bnos Yaakov.