I don't have a problem with teaching about Christianity in school. Here is what I wrote in an earlier post: The spirit in which something is done is crucial. What is appropriate and even essential in a church or a synogogue or a mosque is inappropriate in a public school.
The biggest problem with teaching Chistianity, though, will be: whose Christianity? My guess is, with so many versions of it out there and many of them claiming to be the "true" version (though by no means all of them claim this, thank goodness), that many people who call themselves Christians would be unhappy with whatever would be taught. It is easy to teach Islam in this country precisely because there are still so few of them, relatively speaking.