In these three teshuvot, written over a span of 13 years (1967-1980), Rav Moshe Feinstein makes it very clear that boys and girls are to be separated in educational settings as much as possible.
In the first teshuva, he quickly asserts that coeducation is forbidden. Even if some communities practice it at younger grade levels due to financial considerations, certainly by Middle School boys and girls should be separated.
In the second teshuva, a school asked about using one building for boys and girls, but with substantial measures taken to have boys and girls on completely separate sides of the building with separate entrances, separate playgrounds, separate cafeterias, and some sort of barrier that rendered the two halves of the building virtually distinct from one another. Having seen the plans, Rav Moshe permitted the building, but with the note that the school should continue to look for an opportunity to build a second building on a different campus for the girls.
In the final teshuva, Beit Yaakov of Baltimore had the opportunity to build a new building alongside a Yeshivat Chofetz Chaim, in a move that would be cheaper than building on their own campus. Rav Moshe flatly rejected this plan, noting that especially in our generation when so many boundaries are ignored, and particular in the area of עריות, it is imperative to keep a distance between teenage boys and girls.