Ann Althouse seems to understand the reasoning behind this... I personally don't see it. A school district is actually barring parents from sitting in on a sex ed class (blood sample required for viewing):

Brian Edwards, a county schools spokesman, said the parents' presence at the sex-ed classes would have a "chilling effect on the educational process."

"If you're in a classroom and you want to have a frank discussion among your peers, with whom you've developed trust, and you're going to have Johnny Smith's mother sitting in the corner, you're not going to be as honest," he said.
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"There isn't anything in the school curriculum that parents should not be able to go and hear for themselves," said Michelle Turner, president of Citizens for a Responsible Curriculum (CRC). "If the school feels that parents shouldn't be in the classroom, then that's a red flag for parents."
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The curriculum, which was slightly revised last month, defines one's sexual identity as including gender identity, which is "a person's internal sense of knowing whether he or she is male or female." The instruction also includes the statement that "most experts in the field have concluded that sexual orientation is not a choice."

Also, households with same-sex parents are identified as one type of nine families. Next to that listing, a new phrase has been inserted as instruction to teachers -- not students. It reads in parentheses: "This should not be interpreted as same-sex marriage."
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The pilot classes will begin Thursday. After the pilot classes conclude later this month, a citizens advisory committee will collect feedback from students, teachers and parents and issue final recommendations to the county school board.
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Teachers are "not providing the resource materials for parents to review," she said. In addition, parents were not allowed to take curriculum copies home.

While I can understand how some people might think this is a good idea... I would be fighting to get in and see what is being taught to my child. This seems to me to be the perfect example of how education in this country is going wrong... and I'm not even a parent! If I were a parent, I'd be damned sure to know what was being taught to my child, and no person is going to tell me that I don't deserve to know, and should not be able to see. That is my child, not that state's.

I especially loved the line where they say parents would be able to provide feedback towards a final recommendation. On what information would they be able to provide feedback?! They can't sit in on the class, they can't review or take home any materials... how do you make an informed recommendation? What a bunch of crap!

To be fair here, parents have been asking for this though. So many parents are so scared to talk to their kids honestly about sex. Well folks, this is the result. Like I said, I'm not a parent. I'd like to think that when the time came, I would be able to talk to my child about this stuff openly and honestly, because frankly it's part of your job as a parent. Mine never did. But no matter what I would choose to tell my child, I would resent any person who doesn't think I should know what is being taught to my child, even if I agreed with it.