Whoa, guys, relax, I think Believer was saying that the reason you can't teach it isn't because it's wrong (which is really what it should be) but because it's religious. It shouldn't be a 1st Amendment issue at all but instead an educational issue.
Why don't we teach ID/creationism? The answer shouldn't be "because it's religious and a violation of 1st Amendment rights." The answer should be "because it's wrong."
The topics also shouldn't be hush hush in class, if they're brought up it's the duty of a good educator to unequivocally say that they're wrong and explain why to anyone who has questions.