Complex sentence: At that point, only the lower school was co- ed. That changed in 1975, when Horace Mann School returned to its roots as a co-educational learning environment and began admitting females to the Upper School.
Split into simpler sentences: At that point, only the lower school was mixed. In 1975, the Horace Mann School returned to its roots as a co-educational learning environment and began admitting girls to the Upper School.