Rav Moshe Feinstein writes clearly that mixed schooling is forbidden “min hadin”. Rav Ovadiah Yosef quotes him and agrees with him. In a tshuva (Igros Moshe, Yore Deah – Chelek 3, Siman 78) he states: ACCORDING TO ALL AUTHORITIES, CO-ED EDUCATION IS ABSOLUTELY FORBIDDEN. In fact, he says that the nature of this prohibition is so basic and simple, that there is absolutely nothing to talk about, and no room for discussion.
Even Rav Soloveitchik zt”l, who himself founded a co-ed school more than half a century ago, did so only because “he had no choice” – not because he held it was permissable. In fact, I heard that Rav Hershel Shachter (who probably more than anyone else knew what the Rav Soloveitchik held) mentions (in one of his writings) that Rav Soloveitchik told him that he was afraid that after 120 years, he would be questioned by the Heavenly court why he participated in the founding of a coed school.
Going to a mixed school is not merely a matter of a “chumra” in tznius. It involves numerous transgressions of Scriptural prohibitions, and is therefore vehemently opposed by all the contemporary poskim.
And it is self-evident that, especially in the teenage years when the hormones are raging, coming into daily, close contact with the opposite gender, as is inevitable in any co-ed school, involves regular transgression of this prohibition for both boys and girls.