Nobody with a dictionary and a brain can classify atheism as a religion, since it is the absence of faith in a God
Non-belief in a god, is not non-belief in a religion, unless you want to define religion solely as a belief in god - that fails with various beliefs that most people would rarely argue are not (many Buddhism forms but there are other.
I would be interested to know how you came to the conclusion that most people without religious belief still believe in God.
Irreligion is not a belief in god, it's not taking a position on religion through complete indifference and often hostility (the I hate god of this thread) to define people who completely don't care and have no interest as atheists is labelling them entirely inappropriately. It's some crazy prescriptivist view of a dictionary definition of a complex label.
So I won't label the irreligious as atheist, simply because I expect most don't care, polls certainly suggest that many are not atheist (polls say ~30% don't believe in a god, but ~53% have no religion, which gives a large number of theistic non-believers) but actually polls on this are of the most unreliable since it's not something that matters to so many.
Which is why of course it's worth teaching, because for the few people it does matter to - and even more so in other countries - an understanding helps. And also of course specifically because of sunday school and the like teaching a single religious view from a young age, it's important to combat that early in primary school.
My personal belief is that this religious teaching is why we have such a tiny, tiny number of religious 18 year olds now, and why it's mostly the very religious parents who opt their kids out of RE.