According to quite a few books I have read, helium belongs to the p-block with reasoning along the lines of "it's an inert gas, so let's clump it together with other noble gases which are in the p-block."
However, on Wikipedia, helium is classified as an s-block element which, to me, makes sense because the last (or so-called "differentiating") electron that is added to helium will occupy an s sub-shell.
But my teachers are adamant that helium is a p-block element, and the fact that our textbook covers helium under the chapter "The p-block Elements" just gives them more reinforcement of that idea.
So: is helium an s- or a p-block element? Could someone provide concrete reasons for and against the block selection?