We're completely unreligious but my daughter still goes to the church with school, was in the nativity, etc.
Long story short: the only people who can effectively "force" religion on a child is the parents themselves. So long as parents are providing context and commentary at home - actually talking to their kids about what they learn at school - they really shouldn't have to worry about indoctrination.
Plus I had a conversation with my daughter about church last week that was bloody hilarious... She told me that you can't see Jesus at church, but there's a guy who gets up to speak and that "he's dressed up as Jesus and pretends that he is Jesus"
. Essentially she thinks that ministers are actors putting on little Jesus-based performances. Awesome. Who would want to miss that?