The Secular Education Network has asked the Human Rights Commission to investigate school bible classes, arguing they breach the Human Rights Act.
It said school bible classes were divisive and many parents were too scared to oppose them.
The network's spokesperson, David Hines, told Radio New Zealand's Nine to Noon programme on Tuesday that the classes created a division between Christians and non-Christians.
"That is highly discriminatory. People should have the right to keep their religion private if they want to so the opting-out arrangement is a bad one, we believe. It's also bad for one single religion to be getting state funding to promote its views to children."
Mr Hines said parents could stop the classes by opting out in large numbers.
New Zealand Principals' Federation President Phil Harding told Nine to Noon principals increasingly had to act as referees between parents and school trustees on the issue.
Mr Harding said there was a middle ground. "It's quite doable to create pockets of practice across a school that cater for a wide range of needs, be it Christian, Muslim or atheistic."