Ignoring it won't make it unenforceable!
Robeyw:
With all due respect to your civic teacher and his/her teaching, the reason you have not been able to find any references to "customarily ignored" laws and the effects on their subsequent enforcement is because there is no such a concept; well, at least not within the annals of the American Jurisprudence as such a concept would reduce the entire legislative process and intent to nothing more than idle ceremony and window dressing.
Such a concept, however, does exist and it is discussed at length and in depth within the confines of International Law and the Law of International Organizations.
And as for the validity of any subsequent enforcement of a customarily ignored law, the best example that I can come up with would be highway driving and the speed limit; boy, talk about a law (speed limit) that is not only customarily, but also routinely, universally, and daily ignored to such a huge degree that we feel unfairly victimized and prejudiced against when trapped and ticketed by the highway patrol. Now, the speed limit laws have been ignored and broken since they were first incepted and they continue to be ignored and broken overtly and as a matter of course by every single one us at any given time.
So, does the fact that hundreds of thousands of drivers ignore the speed limit make it null and void and unenforceable as a matter of law? Of course, not!
But all said and done, that is just my two cents, but you have at your fingertips perhaps THE best and most powerful legal research tool ever created, namely Lexis-Nexis and if I were to suggest a cite to start your search on, it would be this
https://www.law.kuleuven.be/iir/nl/onderzoek/wp/WP25e.pdf.
Happy Researching,
fredrikklaw