I make a huge effort to send my DD to her CoE school.
Every morning I have to leave my home a lot earlier than most other parents; get on the tube with a 5year old, and 2year old + buggy, no joke when you have to fight your way through a busy tube station (in suffocating heat during summer) and then push to get on a train where we're all squashed up like sardines during the rush hour. All very stressful.
I make this effort and put myself through all this ,shunning local shcools, because we are religious and I am willing to make the effort to ensure my daughter attends a school which reflects and practices our values and beliefs.
I certainly would not send my child to a school whose ethos went completely against my personal values and beliefs, hence I cannot understand why you would want to send your child to a religious school when you are an athiest and clearly feel very strongly about the risk of your child being "brainwashed".
Why bother to start "canvassing" other parents to see if others are in agreement with you and start up your own little group within the school when you could, surely just like me,get out of bed earlier, get off your backside and make the effort to send your child to a school which is more appropriate to your principles?
I'm afraid I cannot see what the problem is here. It's pretty much black and white; if you don't want any christian influence in your child's life, don't send them to a CoE school, find and alternative.