Thanks for your advice, everyone! Â And... we caught the culprit! Â
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I did decide to take it to the head of my dept., and he recommended that I address it with the entire class.  (I also called a female colleague to ask her advice, too, since my boss is a wonderful man but has the occasional sexist bias.  She seconded his recommendation -- more strongly -- so I figured I was good to go.) Â
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I printed out the relevant section in the university sexual harassment policy and brought it into class. Â We were going over the syllabus that day, so when we got to the section on "Respect in the Classroom" (yes, I have an entire section dedicated to it) I put the sexual harassment policy on the overhead projector and explained what had happened at the previous class. Â You could have heard a pin drop in there. Â I was sure I'd hear a bunch of nervous titters when they heard what the note said, but there was not a sound in the classroom. Â This was heartening; it was clear that they were taking this very seriously.Â
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I gave them quite a lecture, which boiled down to: someone in this room is a creepy little schmuck, so y'all had better be careful until we know who it is.  And when I find out who it is, the Sexual Harassment policy will apply and the consequences are likely to be severe.  And then I looked each of them in the eye and said, "All of you -- but I'm speaking especially to the women in the room -- I want you to know that you should never, ever accept this shit from anybody."
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So I thought that would be the end of it, but after class an extremely embarrassed-looking young man came up to my desk and stammered, "Um, professor, about that note..." Â Apparently, he had written it -- but not to me. Â He had passed it to a buddy across the room, and somehow it was passed forward to me. Â Apparently the phrase is a long-running joke between the two of them, an inside joke whose origins I'm sure I don't want to know. Â
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So, all's well that ends well... I suppose.  I suspect that the buddy did pass it forward deliberately, as a lark, so I'm fairly certain that my lecture was at least somewhat warranted.  And... the women in the class had the opportunity to see a woman standing up for herself. Â
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And perhaps, this week, I'll actually be able to teach them something about the subject of the course!!! Â