I just have to quote Kevin Drum here:
I myself would not argue that Darwinism in biology classes is protected by the free exercise clause of the First Amendment. Rather, I would argue more narrowly that everything else is forbidden. If a school district decides not to teach biology at all, that’s fine. But if they do teach it, they aren’t allowed to include religious proselytizing in the curriculum.
The distinction here is this: creationism is Christian proselytizing, a no-no for government bureaucrats. Intelligent Design is so clearly a thinly veiled version of creationism that it’s forbidden too. Darwinism, however, is simply science. School districts are free to stop teaching science if they want, but if they do teach it, they have to teach Darwinism just as much as they have to teach Newtonian mechanics, Boyle’s law, and the theory of relativity.
The rest of the post is about something else, but still an interesting read.