Is teaching creationism illegal in the USA?

Yes, it's illegal. However, that doesn't mean it isn't done, it just means those schools haven't gotten sued yet. It's not illegal to teach because it's not science (it's not illegal to teach about UFOs or fairies or vampires, just not in the curriculum), it's illegal to teach because it's religion. Intelligent design has been ruled to be the same thing as creationism (because it is) and therefore cannot be taught either.