Atheism isn't a religion, but it is a position on religion.
An atheist may not believe in god, but all of them have a position on whether religion belongs in a science class, for instance.
You can opt out and say "I don't care if biology is taught using creationism or evolution", but opting out means you are ok with creationism being taught.
And if you're ok with that, fine (I mean, you're wrong, but you're entitled to an opinion). But you can't pretend you haven't taken a position on it.
Kind of like saying atheism is a religion.