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Actually, I think what would happen is the teacher would stand up in front of the class, announce that she got the note and then harrangue them on why they shouldn't be embarassed by discussions of "normal and healthy sexuality."
I've had to deal with a lot of people like this and usually they only respond to a firm smackdown (verbal, of course) delivered by someone with the power and authority to enforce it. A note from the kids wouldn't be enough. I don't even think a discussion with another teacher would be enough. It will have to come from the higher ups, which is why I think the LW needs to kick the problem upstairs.