Well, many schools DO offer religion class; that should mean, though, that it isn't strictly devoted to teaching Christianity, but teaching all sorts of religions. I took it for a social studies requirement in middle school and high school. I learned a lot about Buddhism, Islam, Christianity, Judaism, and Hinduism.
And I don't know about you, but WE didn't have to do the pledge of allegiance.
It's not the idea of teaching any religion in schools; it's about preaching it, as opposed to teaching the facts about it. When you start preaching a religion in a federally-sponsored institution, then it gets unconstitutional.