Teaching principles consistent with a religion is tantamount to teaching the relion. Second, atheism most certainly is a religion - it's a belief system wherein one chooses to believe that there is no God. It's a faith belief as much as theism is. Atheism (word comes from theos) cannot exist without theism, and it's adherents spend endless hours arging against theism which is prima facie evidence that there is no absence of belief, but vehement belief AGAINST theism. If atheism is total lack of belief, then a true atheist can only remain silent on the subject of God and theism. I have never met one who can keep his mouth shut in this regard.
Second, atheism is not a religion. It's the abscence of theism, nothing more and nothing less.
This may be beside the point of your reply (and the entire thread), but the real way that a school enforces a particular worldview is by the values it implies. Government-run schools, for example, do not teach any sort of 'higher meaning' to human existence and end up impressing upon the minds of students that the goal of life is external success - in other words, materialism. At the same time, with no demonstration of a basis for their authority beyond mere force, the 'value' of 'might makes right' is also implied.
So, by implying a particular value system (and discrediting others by implying that they're unworthy of consideration), students end up searching for a worldview to accomodate the the value system they've spent thirteen years learning.