How is teaching ID unconstitutional? Isn't silencing people and not allowing them to teach about something (whether it is right or wrong) unconstitutional?

I suppose we've banned teaching of any ideas that might be arbitrarily classified as "hate material" (critisizing corporations is banned - after all they're "people" too), "pornography" (even so much as a nipple under any context), and "terrorist" (i.e. anarchist or anti-establishment). So under this pretext there's no reason not to ban ID theory/philosophy/whatever...we've already banned free thought and free speech which are more important and the what prevent anything else from being banned anyways.