Intelligent Design is a religious idealogy. Teaching it as a discipline, like science, counts as religious teaching and doctrination. In public schools that counts as a breech of separation of church and state. Only viable teaching, specially when it comes to the origins of the earth and life is one that can be tested by scientific processes. However, as Withteeth states, if taught as a form comparative religions, philosophy, and similar disciplines I would not have a problem of it being taught in public schools.