Are Papal Encyclicals in Effect Unless Revoked (e.g., Acerbo Nimis)?
Not all papal encyclicals deal exclusively on faith and morals. Some deal with ecclesiastical discipline and as such Church discipline can change. Papal encyclicals, of themselves are not infallible documents, encyclicals may (and generally do) contain pronouncements on faith and morals that are de facto infallible because they express the ordinary teaching of the Church. Many deal with Church matters that are open to change with times.
A papal document treating of matters related to the general welfare of the Church, sent by the Pope to the bishops. Used especially in modern times to express the mind of the Pope to the people. Although of themselves not infallible documents, encyclicals may (and generally do) contain pronouncements on faith and morals that are de facto infallible because they express the ordinary teaching of the Church. In any case, the faithful are to give the papal encyclicals their interior assent and external respect as statements of the Vicar of Christ.
An encyclical epistle is like an encyclical letter but addressed to part of the Church, that is, to the bishops and faithful of a particular area. Its contents may be doctrinal, moral, or disciplinary matters of universal significance, but may also commemorate some historical event or treat of conditions in a certain country or locality.
Thus, the quote you stated from the papal encyclical Acerbo Nimis is dealing with a Church discipline; a future pope is free to alter it or revoke it completely.
On every Sunday and holy day, with no exception, throughout the year, all parish priests and in general all those having the care of souls, shall instruct the boys and girls, for the space of an hour from the text of the Catechism on those things they must believe and do in order to attain salvation.
Times are clearly different now than in the times of Pope St. Pius X. Priests have larger parishes. There are fewer priests. Some priests I know say four Masses on Sunday. So for priests to teach children on Sundays for an hour is almost a physical impossibility. Some parishes of olden days may have had 30-40 families. Now with less priests, they may have hundreds or maybe a thousand or more families.
In other words, your quote is obsolete in our days. The Church has had to adapt to new circumstances.
Pope St. Paul VI in his document on Christian education Gravissimum Educationis outlines many issues facing Catholic education and goes into details about Catholic Schools in our times.
The portion of Acerbo Nimis that you quoted does not have to be explicitly revoked to be obsolete. Simply try looking up the most up to date Vatican norms on this issue to understand the Church’s position at any given time.