All Catholics are under the pain of mortal sin to attend Mass every Sunday and all Holydays of Obligation (Christmas, Assumption, Ascension, All-Saints, etc.). This is Catholicism 101, like making the Sign of the Cross, genuflecting to the tabernacle, and knowing the Hail Mary.
The Council of Elvira, around AD 300 made the canon: "Canon 21. If someone living in a city does not come to Church for three Sundays, fasting is to be imposed on him for a short time, until he is seen to have reformed."
If you can't be bothered to attend Mass weekly, you are not Catholic. People who refuse to practice the rudiments of their faith, when it is widely and easily available are called APOSTATES.
Second, were the parents informed of the attendance tracking via the envelopes' bar coding? If so, were they able to verify their attendance by some other method (ex: what happens if you forget your envelope and just drop cash in the basket)? Could the student just inform a deacon, teacher, or lay assistant that they are there?
Having taught these classes, I found it easy to both ask and verify if students had attended. They were all very honest about it, and you could easily tell, since we asked nothing more than to remember the simplest point of one of the three readings or the sermon, or what moment was being commemorated (as on Palm Sunday, of the Baptism of the Lord, or the beginning of Lent or Advent). There would definitely be no mistaking who was and was not there.
Third, if parents were aware of the class requirement, why weren't they making some other arrangements with the priest if family pressures or obligations were interfering with their attendance.
Because they are CINO - Catholic In Name Only. They believe First Communion parties are the birthright of their children, regardless of their own beliefs or personal life.
Considering the probable age of the parents, it's more than likely they were never taught these things -- some may have even have been told utterly otherwise from the pulpit. (Someone I knew in the 70s asked a priest for a dispensation from abstinence on Good Friday. She was told no, she could give herself a dispensation.) The wonder is that they retained enough to send their kids to CCD at all.