I would agree that this is a bad interpretation of what the common core says. Common core is not a curriculum, but a set of skills/goals that curriculum are supposed to be designed around so that all high school graduates from the US will have the same set of "common core" skills, regardless of the actual content they have been taught.
If the principal accepted the teacher's answer that it was because of the common core that she was doing this, then he needs to look up the common core himself and get a better understanding of them.
