79 From this it follows that the so-called "neutral" or "lay" school, from which religion is excluded, is contrary to the fundamental principles of education. Such a school moreover cannot exist in practice; it is bound to become irreligious. There is no need to repeat what Our Predecessors have declared on this point, especially Pius IX and Leo XIII, at times when laicism was beginning in a special manner to infest the public school. We renew and confirm their declarations, as well as the Sacred Canons in which the frequenting of non-Catholic schools, whether neutral or mixed, those namely which are open to Catholics and non-Catholics alike, is forbidden for Catholic children, and can be at most tolerated, on the approval of the Ordinary alone, under determined circumstances of place and time, and with special precautions. Neither can Catholics admit that other type of mixed school, (least of all the so-called "école unique," obligatory on all), in which the students are provided with separate religious instruction, but receive other lessons in common with non-Catholic pupils from non-Catholic teachers.
Source: Divini Illius Magistri (Vatican.va)