I dont really see why teaching christianity (or any other religion) is such a big deal. It mostly depends on how it is being given to students.
I can see that opinions about it might be a bit spreaded if it is being thaught by a pastor, or a teacher who is generally interested in the subject. According to wikipedia more then 4 billion people believe a major religion, neither is it a subject you never come across. While at school I often get questions about christianity which they would know if they ever looked up something on the internet, or if they would have had some lessons about it.