The first amendment of the Constitution of the United States contains this line:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;...
The importance of these lines is monumental to the matters of Church and State. According to the first amendment, the government, and by extension, the state itself, may make no law respecting any establishment of any religion. For example, the government, by law, may not force any one individual or a group of individuals to practice any religion, and it must not allow any religion to interfere into governmental, and by extension, state matters.
Therefore, Creationism cannot be taught in schools because it is explicitly religious.