Who said anything about arguing with other kids? Not I!
Having the kids talk about religion among themselves may get them to go home and talk about religion with their parents. This is a good thing. When I was a kid I came home from Catholic school and told my Dad that my friend Bernardette said that the Jews killed Christ. My Jewish Dad explained to me that the Romans in fact killed Him, and we read commentary on the subject. Then he told me not to go back and be rude to Bernardette because she was only saying what her parents said and maybe they didn't know any better. He said to explain to her what we'd read in the Bible instead. And by the way, to quit calling her a "mackerel snapper."
If kids talk about religion in school, it can only be good.