Single sex schools worsen existing gender stereotypes
These schools entrench gender stereotypes because only subjects with enough popularity among the school's students will be taught. Therefore, subjects taken by the opposite gender do not have enough popularity at a single sex school to justify that school teaching that subject.
Students are from very young age on being conditioned that their gender is so important that they better be taught in different schools.
Schools are meant to prepare students for the workplace and that is generally not gender-segregated.
Single sex schools make students more comfortable to step out of gender stereotypical roles as they don't feel pressured to take gender-stereotypical subjects by their peers