Preaching classes would create littl' gangs in school, and with today's tension it would turn into a christian vs muslim high school war, especially if christian got a preaching class and not muslims (which would be the case in USA). I can see from here the 32 fundamentalist christian who met in their religious freaky class beating the 3-4 muslims and the only zoroastrian in the whole high school.
Imagine we create, let's say, a christian preaching class. If you are a moderate christian, the typical John Smith, and your teacher is a hardcore heebee-jeebee fundie (or vice-versa, who care...), will you get a bad mark for accepting Darwin until you became creationist? If the teacher is catholic and you are a baptist, will you get a bad mark for not loving the Pope?
My point is, religion is too personal to be a school subject, and in some back country places it could derivate into an endoctrination center where personal issues (exemple: a christian fighting with a jew, for non-religious reasons) can become a theologic conflict (he attacked one of us, let kick his christian ass!).