“(to prove my point, how many very very high up private goyish schools have separate classes? a lot, exactly because of this.) “
True indeed, but the SCHOOLS are not necessarily separate. The boys and girls have mixed events, might lunch together, have mixed school activities (actually called MIXERS)in the form of extra curricular activities. They just do not sit in classrooms together. They are also very young. When kids are in college and/or grad school, presumably they have matured sufficiently to know it’s important to become educated in school. And if not, then they are not ready to get married, anyway, even were they only in single gender schools.