CSDC 240. Parents are the first educators, not the only educators, of their children. It belongs to them, therefore, to exercise with responsibility their educational activity in close and vigilant cooperation with civil and ecclesial agencies.
See below from the Vatican Website two different translations of the same quote. Apparently both translations are possible (rightly understood), but the compendium quote agrees with only a weaken sense of “primary” which cannot be interpreted as “sole.” It also requires translating the same word primus two different ways in the same paragraph?
36. Hence, parents must be acknowledged as the first and foremost educators of their children. Their role as educators is so decisive that scarcely anything can compensate for their failure in it. For it devolves on parents to create a family atmosphere so animated with love and reverence for God and others that a well-rounded personal and social development will be fostered among the children. Hence, the family is the first school of those social virtues which every society needs."(Gravissimum Educationis, 3)
Gravissimum Educationis, 3
The Authors of Education
Since parents have given children their life, they are bound by the most serious obligation to educate their offspring and therefore must be recognized as the primary and principal educators.(11) This role in education is so important that only with difficulty can it be supplied where it is lacking. Parents are the ones who must create a family atmosphere animated by love and respect for God and man, in which the well-rounded personal and social education of children is fostered. Hence the family is the first school of the social virtues that every society needs.
11. Cf. Pius XI's encyclical letter Divini Illius Magistri, 1, p. 59 ff., encyclical letter Mit Brennender Sorge, March 14, 1937: A.A.S. 29; Pius XII's allocution to the first national congress of the Italian Catholic Teachers' Association, Sept. 8, 1946: Discourses and Radio Messages, vol. 8, p. 218.I checked Pius XI's encyclical letter Divini Illius Magistri, 1 and it does not say either first or primary. My German is a bit rusty … Litt. Encycl. Mit brennender Sorge, 14 martii 1937: AAS 29 (1937), p. 164 the rest is hard to track down.
Latin Text Gravissimum Educationis, 3
3. Parentes, cum vitam filiis contulerint, prolem educandi gravissima obligatione tenentur et ideo primi et praecipui eorum educatores agnoscendi sunt.
Quod munus educationis tanti ponderis est ut, ubi desit, aegre suppleri possit. Parentum enim est talem familiae ambitum amore, pietate erga Deum et homines animatum creare qui integrae filiorum educationi personali et sociali faveat. Familia proinde est prima schola virtutum socialium quibus indigent omnes societates. Maxime vero in christiana familia, matrimonii sacramenti gratia et officio ditata, filii iam a prima aetate secundum fidem in baptismo receptam Deum percipere et colere atque proximum diligere doceantur oportet; ibidem primam inveniunt experientiam et sanae societatis humanae et Ecclesiae; per familiam denique in civilem hominum consortionem et in populum Dei sensim introducuntur. Persentiant igitur parentes quanti momenti sit familia vere christiana pro vita et progressu ipsius populi Dei.