Teachers (or school systems) that mandate this system, and, even worse,
GRADE NOTEBOOKS completely misunderstand the point of it. In K-12, one of
the goals is to help students become effective learners, which involves
students identifying the tools that help them learn best.
A single assignment forcing Cornell Notes (or any other note taking system) might be all right. Something like, "Here's a reading, please submit a summary of it in Cornell Notes format." That gets the kids to interact with the system, and it exposes students to something that they might find useful going forward. To force its use for an entire quarter or semester just sounds like sadism.
Furthermore, from what I can tell, the Cornell system is highly tied to paper (and lined letter/A4 sized paper at that), and is basically useless outside of a school/university context. It doesn't do much to structure your notes. Instead, the class itself is assumed to be providing the structure, and the notes combine with the class structure to provide an outline of your knowledge.
In the "real world", where information is coming at you from a dozen different sources, and it's up to you to provide your own structure, something like Cornell falls down, hard.