R. Tzvi Yehuda Kook in R. Shlomo Aviner, Gan Na’ul, pp. 32-33:
Mixed swimming is the most severe example of intermingling between the sexes but coeducation also has no connection to normal Judaism. In normal, healthy, kosher Judaism, who knew of such a thing? Boys learned in a heder and girls alone in a school. When we came to Israel, there were economic difficulties on the Moshavs which led to people becoming used to mixed schools. But coeducation as a matter of principle – there was never such a thing. Mixing boys and girls is not a simple matter. It is against human nature and the halakhos of tzeni’us.