In single-sex schools, students may have fewer opportunities for formal social interaction with members of the opposite sex during their school years.

the 是定冠词，用于特指某一类人或事物。 opposite 的意思是"相反的、对面的"，在这里指性别上的"相反的"。 sex 的意思是"性别"，指生物学上的男性或女性分类。 the opposite sex 是一个固定短语，指"异性"，即与自己性别不同的人群。