Since I had the same reaction as Nick:
Sexism as such is not funny, a mildly* sexist joke that is conscious of its own sexism can indeed be funny, especially in the context of a serious discussion about a teacher's bad moods that does not take the teacher's gender into account (and rightfully so).
[*] 'mildly' because it does refer to a stereotype, but does not imply female inferiority.