I remember reading through the textbooks and noticing big glaring gaps, like, everywhere. But if you ask the instructor about it, or information that comes from outside of the textbooks, they often get nervous or just state, "it's not part of the approved curriculum". That was my experience in the U.S. in the 90s to early 2000s.

I also didn't think much of it at the time, but I remember the principal and other reps from the school board would periodically sit in on the clear to verify that the approved curriculum was being taught.