“Those Frum schools that do that because of the parent clientele. It’s not ideal. It’s usually Kiruv types of schools!”
In my experience, it has nothing to do with ‘parent clientele.’ It has do do with the cost of opening/running a school in a small frum community.
It may not be your ‘ideal.’ but ideal for the particular community.
Most OOT communities don’t have Kiruv type of schools and Non-Kiruv type of schools. They have an orthodox day school, period. Many run by Chabad, others part of Torah Mesorah
The Day school in the New Haven area that is mixed gender in the primary grades has existed for almost 75 years. Their Girls High School for 50, BUT there have been 4 Boys high Schools in the past 50 years that have failed. Families want their little kids and girls at home, they are willing to send teen boys away for high school.