The Catholic Church has always stressed the essential need for parents to send their children to Catholic schools unless there is no other possible option. Below are teachings from 5 different Popes on the subject:
Catholic Church Teaching on Education:
The Church cannot approve schools which exclude religion from the curriculum, both because religion is the most important subject in education, and because she contends that even secular education is not possible in its best form unless religion be made the central, vitalizing, and co-ordinating factor in the life of the child. The Church, sometimes, tolerates schools in which religion is not taught, and permits Catholic children to attend them, when the circumstances are such as to leave no alternative, and when due precautions are taken to supply by other means the religious training which such schools do not give. She reserves the right to judge whether this be the case, and, if her judgment is unfavourable, claims the right to forbid attendance" (Letter of Gregory XVI to Irish Bishops, 1831).
"...Those parents who allow their children to frequent schools where it is impossible to avoid the loss of souls… according to Catholic moral teaching, such parents, should they persist in their attitude, cannot receive absolution in the Sacrament of Penance." Instructions Of The Holy Office To The Bishops Of The U.S., Pope Pius IX, 1875
"It is, then, incumbent on parents to strain every nerve to ward off such an outrage, and to strive manfully to have and to hold exclusive authority to direct the education of their offspring, as is fitting, in a Christian manner, and first and foremost to keep them away from schools where there is risk of their drinking in the poison of impiety" Sapientiae Christianae, Pope Leo XIII, 1890
"First, Catholics should not choose mixed schools but have their own schools especially for children. They should choose excellent and reputable teachers for them. For an education in which religion is altered or non-existent is a very dangerous education" Militantis Ecclesiae, Pope Leo XIII, 1897
"Obviously the need of this Christian instruction is accentuated by the decline of our times and morals. It is even more demanded by the existence of those public schools, lacking all religion, where everything holy is ridiculed and scorned. There both teachers' lips and students' ears are inclined to godlessness. We are referring to those schools which are unjustly called neutral or lay. In reality, they are nothing more than the stronghold of the powers of darkness." Editae Saepe, Pope St. Pius X, 1910
"Duty of Attending Only Catholic Schools. Catholic children may not attend non-Catholic, neutral, or mixed-schools, that is, those which are open also to non-Catholics, and it pertains exclusively to the Ordinary of the place to decide, in accordance with instructions of the Holy See, under what circumstances and with what precautions against the danger of perversion, attendance at such schools may be tolerated (Canon 1374).
"1. Neutral schools are those which exclude religion by p rescinding from it, such as the public schools in the United States. Mixed schools are those which admit pupils of any or no religion. Catholic schools, however, even though they admit some non-Catholic pupils, do not come under this classification.
"2. Does the provision of canon 1374 apply only to elementary and high schools, or also the colleges and universities?
"a. The natural law itself forbids Catholics to attend schools, whatever their grade, if they are dangerous to faith or morals. Both common experience and many documents of the Holy See prove that this danger may exist not only in the elementary and high school but in college and university as well. (As to elementary and high schools, especially the public schools in the U.S., see Instruction of the Holy Office, 24 Nov., 1875. As to colleges and universities, see S.C. Prop. Fid., 7 Apr. 1860; Fontes, n. 4649, Vol. VII, p. 381, and earlier documents there cited; also S.C. Prop. Fid., 6 Aug. 1867; Fontes, n. 4868, Vol. VII, p. 405.) 'It is almost if not quite impossible for those circumstances to exist which would render attendance at non-Catholic universities free from sin.' (S.C. Prop. Fid., 6 Aug. 1867; Fontes, n. 4868, Vol. VII, p. 405.) It was in regard to universities that the Holy See declared: 'The unformed and unstable characters of young people, the erroneous teaching which is inhaled as it were with the very atmosphere in those institutions without being offset by the antidote of solid doctrine, the great power exerted over the young by human respect and the fear of ridicule on the part of their fellows--all these things produce such a present and proximate danger of falling away, that in general no sufficient reason can be conceived for entrusting for entrusting Catholic young people to non-Catholic universities.' (Encyclical of the S.C. Prop. Fid., to the Bishops of England, 6 Aug. 1867; Fontes, n. 4868, Vol. VII pg. 405.)
"b. The only thing which this canon adds to the obligation of the natural law is the provision that it is for the Ordinary of the place to decide in accordance with the instructions of the Holy See, under what circumstances and with what precautions against the danger of perversion, such attendance may be permitted... Does it apply equally to colleges and universities? We think that no such strict canonical requirement can be proved... In the absence of such legislation, parents and young people are bound by the natural law to remove effectively the danger of perversion by employing safeguards which are really sufficient. It is prudent and advisable, not strictly obligatory, to consult the Ordinary on the sufficiency of these precautions." From Canon Law: A Text and Commentary, by Bouscaren and Ellis (1951, pgs. 762-4)
"Another very grave danger is that naturalism which nowadays invades the field of education in that most delicate matter of purity of morals. Far too common is the error of those who with dangerous assurance and under an ugly term propagate a so-called sex-education, falsely imagining they can forearm youth against the dangers of sensuality by means of purely natural, such as a foolhardy initiation and precautionary instruction for all indiscriminately, even in public; and, worse still, by exposing them at an early age to the occasions, in order to accustom them, so it is argued, and as it were to harden them against such dangers. Such persons grievously err in refusing to recognize the inborn weakness of human nature, and the law of which the Apostle speaks, fighting against the law of the mind; Rom., vii, 23. and also in ignoring the experience of facts, from which it is clear that, particularly in young people, evil practices are the effect not so much of ignorance of intellect as of weakness of a will exposed to dangerous occasions, and unsupported by the means of grace.: Pope Pius XI on Sex Education, 1929
And first, as regards family life, it is of the highest importance that the offspring of Christian marriages should be thoroughly instructed in the precepts of religion; and that the various studies by which youth is fitted for the world should be joined with that of religion. To divorce these is to wish that youth should be neutral as regards its duties to God; a system of education in itself fallacious, and particularly fatal in tender years, for it opens the door to atheism, and closes it on religion" ON THE RELIGIOUS QUESTION IN FRANCE, Encyclical of Pope Leo XIII promulgated on February 8, 1884
Looking at Church teaching, parents have a serious obligation for seeing to proper Catholic education of their children. Catholic children must always be sent to Catholic schools unless not otherwise possible.
And as we can see from the commentary on Canon Law, even adults must use extreme caution when attending non-Catholic colleges and many courses teach contrary to Catholicism, and are occasions of sin. If we look at our page on "Occasions of Sin", it is itself a sin for us to knowingly put ourselves in the occasion of sin.