You can try to convince her by saying that it's better to go to a mixed school
Some parents that had to go to an only boys/only girls school now know that this kind of education is no longer concieved as positive and if they could they would chose to not have gone there.
Basically, education on this kind of schools (at least on my country) are supported by christian groups and have a sexist education, and they have a skirt uniform for girls, and only long trousers for boys and all this.
