Science is a study of nature. But what is nature? For someone who believes in God, nature is God's creation, and therefore science is the study of God's creation, and therefore you can't study God's creation and exclude the process of creation as revealed by the Creator to Moses in Genesis.
But since existence of God can't be proven or disproven, it would be scientifically incorrect to say that God exists. Science is the study of the observable physical world. God is not observable, nor is He physical. So the study of God is called theology and it is not science.
With that perspective, creationism is not pure science, but a combination of science and theology. And since theology is banned from schools, so is creationism.
Scientific method is taught at school, or at least they somewhat try. In my opinion, they should teach the scientific method, and they should teach that there is a THEORY of macro-evolution, but it's just a theory, because it can't be observed or reproduced in a lab, and that other theories exist such as creation, and that evidence exists to support both, but there is not enough evidence to definitively prove one or the other. And that in fact you can't prove creation with science, because as stated above, you can't prove the existence of Creator with science.