Co-education

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 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.