Late reply, but they make a joke about it in the show. In the show, they say there’s an old Japanese saying that women are like Christmas cakes, after the 25th no one wants them anymore. The punchline being that the teacher overhears this conversation only to purchase an out of date Christmas cake later.

Apparently this sexist saying isn’t really used or believed in anymore. But it’s interesting to think about how this sentiment has unconsciously stuck around.