Every once in a while when my daughter tells me about her school experiences, I am amazed at some of the things her teachers say to the students. If you had asked me a few years ago, I would have said that professors feel less restricted about the expressions they use (idioms, similes, metaphors, synecdoche, preterition) when teaching than do K-8 teachers, but now I think otherwise.
Example: When students turn in homework or exams with handwriting that is impossible or difficult to decipher, making the already-painful task of grading even more painful, I talk to the class, explain the importance of writing in such a way that I can read what they have written, and gently threaten to assume that an answer is wrong if I can't read it. This typically works to improve handwriting to a legible, if not beautiful, level. In extreme cases, I write illegible comments on student work, and when the student comes to talk to me about my poor handwriting, we chat about the importance of legibility.
My daughter's teacher, however, yelled at her class last week and told them that they all write like "chickens on crack". My daughter and her friends thought that was very funny and they have been joking about chickens on crack for days. The teacher also made some other comment involving reference to "meth", and now there is a group of pre-teens who think meth is funny.
I am not upset about the teacher's casual references to drugs, but I am surprised at her use of these expressions in class. I attempt to make jokes in my classes, but so far have not had occasion to joke about drug use by humans or barnyard animals. I asked a few professor colleagues if they would make a similar joke, and all said no. It's not that we think our students will descend into a self-destructive abyss of drug addiction if we make a drug-themed joke in class, but somehow it just doesn't seem appropriate.
I suppose one use of unusual expressions -- perhaps even somewhat shocking expressions -- is to get students' attention and to make your point especially memorable. If you say something that the students don't expect, maybe what you say will have a greater impact. This would be one situation in which "chickens on crack" type expressions would come in handy, whether you are teaching at an elementary school or a university.
It is somewhat tempting to do an experiment. In one class, politely request that the students write in a legible way, calmly stating that it would be really nice if they made an effort with penmanship so that you could read their assignments and exams more easily. In another class, raise your voice and tell the students they write like "chickens on crack". Would there be any difference in level of improvement of penmanship of the two classes?
4 hours ago