35 On this point the common sense of mankind is in such complete accord, that they would be in open contradiction with it who dared maintain that the children belong to the State before they belong to the family, and that the State has an absolute right over their education. Untenable is the reason they adduce, namely that man is born a citizen and hence belongs primarily to the State, not bearing in mind that before being a citizen man must exist; and existence does not come from the State, but from the parents, as Leo XIII wisely declared: "The children are something of the father, and as it were an extension of the person of the father; and, to be perfectly accurate, they enter into and become part of civil society, not directly by themselves, but through the family in which they were born." "And therefore," says the same Leo XIII, "the father's power is of such a nature that it cannot be destroyed or absorbed by the State; for it has the same origin as human life itself." It does not however follow from this that the parents' right to educate their children is absolute and despotic; for it is necessarily subordinated to the last end and to natural and divine law, as Leo XIII declares in another memorable encyclical, where He thus sums up the rights and duties of parents: "By nature parents have a right to the training of their children, but with this added duty that the education and instruction of the child be in accord with the end for which by God's blessing it was begotten. Therefore it is the duty of parents to make every effort to prevent any invasion of their rights in this matter, and to make absolutely sure that the education of their children remain under their own control in keeping with their Christian duty, and above all to refuse to send them to those schools in which there is danger of imbibing the deadly poison of impiety."
Source: Divini Illius Magistri (Vatican.va)