I also had the same problem happen to me in the mid-90's in a DISD school. The class also happened to be Chemistry AP.

Most of the students failed to or didn't want to understand the subject matter and were failing.

But I do not think it was due to the inability of this teacher.

Numerous steps were taken including other students helping, on a one on one basis, those that did not understand.

It was because those students did not belong in this class but somehow made it. I suspect a teacher from a prerequisite course gave them an easy passing grade.

I suspect this has happened many times over and do not think that this situation is that rare.