Well, here in Argentina our Geographie books tell us that America is a continent, and it can be subdivided in subcontinents (northamerica, central america, and southamerica). And it sounds logical to me.
I always heard about 5 continents (never 6, and if, it was because antarctica)... I try to remember references outside my country pointing to any position, and its always America as a unity... 5 rings in the olympics
Internet should have an answer, but it seems its not so clear there either:
to stay on topic...