That study simply looks at the differences between students at single- and mixed-sex schools. There are so many confounders I doubt it can be used to say someth

That study simply looks at the differences between students at single- and mixed-sex schools. There are so many confounders I doubt it can be used to say something meaningful purely about the effects of sex separation. They do (try to) control for some confounders but they are all related to the students' individual backgrounds and none of them related to e.g. the school or the teachers. And the schools are all Catholic.