Thankfully we don't need to pass a law to make it illegal to teach anything but evolution. It's legal to teach evolution, there's a requirement to teach science in science class, and teaching religion is not legal. So we're only left with evolution. Although, some of the latest sneaky laws from the right are always trying to set that asunder. The current idea is to non-discrimination against religious viewpoints law attempted in a number of southern states. The idea is basically that you make it illegal for schools to have people suffer grade-wise for their religious beliefs. If you are asked what the age of the Earth is and you write down 6,011 (4004 + 2008 - 1) you're right, regardless that it's closer to 4.5 billion years old (.000133% as much as you should have answered). They don't get you marked as wrong. Some say teachers can teach other views as well.
I predict that under such a law, it would be a near requirement for students to promptly convert... to discordianism.
Malaclypse the Younger: Everything is true.
Greater Poop: Even false things?
Mal2: Even false things are true.
Greater Poop: How can that be?
Mal2: I don't know, man, I didn't do it.