I've been meaning to read that book rather than just skimming it.
'False also and harmful to Christian education is the so-called method of "coeducation." This too, by many of its supporters, is founded upon naturalism and the denial of original sin; but by all, upon a deplorable confusion of ideas that mistakes a leveling promiscuity and equality, for the legitimate association of the sexes. The Creator has ordained and disposed perfect union of the sexes only in matrimony, and, with varying degrees of contact, in the family and in society. Besides there is not in nature itself, which fashions the two quite different in organism, in temperament, in abilities, anything to suggest that there can be or ought to be promiscuity, and much less equality, in the training of the two sexes. These, in keeping with the wonderful designs of the Creator, are destined to complement each other in the family and in society, precisely because of their differences, which therefore ought to be maintained and encouraged during their years of formation, with the necessary distinction and corresponding separation, according to age and circumstances. These principles, with due regard to time and place, must, in accordance with Christian prudence, be applied to all schools, particularly in the most delicate and decisive period of formation, that, namely, of adolescence; and in gymnastic exercises and deportment, special care must be had of Christian modesty in young women and girls, which is so gravely impaired by any kind of exhibition in public. Recalling the terrible words of the Divine Master: "Woe to the world because of scandals!" We most earnestly appeal to your solicitude and your watchfulness, Venerable Brethren, against these pernicious errors, which, to the immense harm of youth, are spreading far and wide among Christian peoples. '
Pope Bl. Pius IX
'Moreover, not content with removing religion from public society, they wish to banish it also from private families. For, teaching and professing the most fatal error of "Communism and Socialism," they assert that "domestic society or the family derives the whole principle of its existence from the civil law alone; and, consequently, that on civil law alone depend all rights of parents over their children, and especially that of providing for education." By which impious opinions and machinations these most deceitful men chiefly aim at this result, viz., that the salutary teaching and influence of the Catholic Church may be entirely banished from the instruction and education of youth, and that the tender and flexible minds of young men may be infected and depraved by every most pernicious error and vice. For all who have endeavored to throw into confusion things both sacred and secular, and to subvert the right order of society, and to abolish all rights, human and divine, have always (as we above hinted) devoted all their nefarious schemes, devices and efforts, to deceiving and depraving incautious youth and have placed all their hope in its corruption.'
Pope Bl. Pius IX, 'Quanta Cura'