My daughters had T”H classes in 12th grade, given by a terrific rebbetzin in the community. They learned basics, not intensively as in a true kallah class, but the basic hashkafos, the basic halachos and harchakos, and so forth. It was a VERY GOOD idea, and made it so much easier for when the girls who were getting married did take real kallah classes.
As to the idea that what do the girls have to know about their parents’ private issues – are you serious? Do you really think that when a girl becomes a young woman, she does not have the seichel to understand that her mother might be a niddah at different times? The only caveat is that the mother needs to try to be very discreet about when she goes to the Mikvah, but teenage daughters are not stupid. I know women who b’davka keep a bag of some innocuous grocery item (canned goods or detergent) in a bag in the car, so she can “go shopping” for an hour and come back with it. But please do not underestimate our girls’ intelligence.