Okay, let's use your definition:
Wikipedia: "Religion is a collection of cultural systems, belief systems, and worldviews that establishes symbols that relate humanity to spirituality and, sometimes, to moral values."
Based on your comment above, you agree that Atheism is "a collection of cultural systems, belief systems, and worldviews" which seems appropriate given that only "cultural systems" is questionable.
"moral values" is behind a sometimes, so is not required to meet the definition.
So, the main open question is, does Atheism "relate humanity to sprituality?"
Atheism most decidedly DOES relate humanity to spirituality. Specifically, Atheism includes a belief that humanity created spirituality to serve its own interests. This is not the relation proposed by most religions, but Atheism isn't most religions.
NOTE: I'm unclear on what "establishes symbols" means in the context of the Wikipedia definition. If it is essentially referring to ritual, I suspect many Eastern religions would not fit this definition. The definition holds without it, so I'll choose to consider it non-operative for now. Parent also did not argue this point.
Does Atheism require a person to go to Atheist School on Tuesday mornings? no. There is no ritual in Atheism. However, many Eastern religions also have very little to no ritual.
Does Atheism require a belief in the Supernatural? no. However, as this is the defining belief of Atheism, that alone seems slim grounds to exclude it.
Do Atheists self-identify with Atheism as their religion? not usually. This is probably the best argument that Atheism is not a religion, but it won't keep others from classifying it as such, just like no matter how much Mormons claim to be Christians, most evangelicals will not accept them. The unsettling part here is that the argument is quite literally: "Atheism isn't a religion because Atheists say it isn't."