JA – it doesn’t have to be that way even though it often is.
I think it has to do with the fact that people look for different things in boys’ schools than they do with girls’ schools. When it comes to boys’ schools, people are looking at the learning styles which is an individual matter. When it comes to girls’ schools, it’s about the hashkafa and type of family.
I’m not saying it’s right or wrong – just explaining why it happens. But you are right that there is something unfair about it.
Even if it does make sense most of the time, if it doesn’t make sense in a particular case, that girl should be allowed to go to a different school than her sisters. And there are families that will send different kids to different schools.