The problem arises with the separation of church and state. Claiming one is true over the other is the same as saying the other is false. And since the government cannot force religious views on you, it simple cannot force the opinion that they are false or one is more right on you either. And that is not even touching the fact that evolution does not disprove creationism whatsoever at all. Nowhere in science does it say that seeing something happen one way means it's the only way possible.
What bollocks.
Following your argument through to its logical conclusion means that if any claim by any religious sect contradicts the school curriculum we could no longer teach that topic. It is not just creationism, by the time you add in the beliefs of the raelians, scientologists and every other cult out there, you'd not be able to teach anything at all. Should we not teach Native American history because the mormons tell us they are a lost tribe of Israel and therefore teaching their real history is now prohibited by seperation of church and state? What a ludicrous concept.
Evolution is the best scientific description we have of the diversity of life we see on the planet and that is why we teach it in science class. Whatever your other views on creationism, it is a wholly religious idea and has no place in the science classroom.
Happiness is twin floppies.