Before the 1960s, many private institutions of higher education in the United States restricted their enrollment to a single sex. Indeed, most institutions of higher education — regardless of being public or private — restricted their enrollment to a single sex at some point in their history. Co-education is a relatively new phenomenon in terms of World History. It is definitely new in the History of the Jewish People. In the Orthodox Jewish communities around the world, the vast majority of children attend Jewish schools at the elementary, high and post high school level. Many schools are "Separate" for boys and girls and many schools are "Co-ed". The Haredi community generally send their children to "Yeshivos" for boys and "Beis Yaakov's" for girls. The Halachic / Hashkafic discussion on this subject is highly debated among Rabbis and lay leaders both in Israel and in the Diaspora. Most of the discussion revolves around the philosophies of modern Jewish thinkers on how to preserve authentic Judaism in the modern age. It is safe to assume that most Rabbis feel that it is definitely NOT correct from a Torah perspective for schools to be co-ed, especially as the children get older. Kiruv Rechokim projects and Jewish Outreach programs are also divided on this matter. Rav Moshe Feinstein, one of the greatest Poskim of the 20th century wrote in a Teshuvah dated in 1954 that there is an obligation of Chinuch to separate schools between boys and girls even at a very young age; even before the children reach puberty boys and girls need to be trained to stay separate from the opposite gender. In a small Jewish community that had two options to either (a) Open a co-ed Jewish school for the boys and the girls or (b) Open a Yeshivah for boys and send the girls to a non-Jewish school Rav Moshe held that option a was better. That was the only case in which he allowed for a co-ed school. In a later Teshuvah written to the Jewish community of Scranton in the 1980s where there were only 81 Jewish children in the entire community in which case separating the boys and girls would result in having 3-5 children in each class, thereby making it extremely difficult to run a normal school, Rav Moshe held that in such a case we say עת לעשות לה' הפרו תורתך and he allowed "only the city of Scranton" to have a co-ed school. In another Teshuvah sent to R. Elya Brudny of the Mir Yeshivah in Brooklyn, NY dated in 1982, Rav Moshe clarifies his ruling (made public by his grandson R. Mordechai Tendler) that schools are obligated to separate boys from girls from the ages of 4 and 5 in order to impress upon the children's minds and hearts the importance of staying separate from the opposite gender. When asked if a school that separates from age 4 is better than a school that separates from age 5, Rav Moshe said "Not necessarily".
These Teshuvot and more on the subject are printed in Igrot Moshe Yoreh Deah 1:137, 2:104, 3:73, 3:78, 4:28.