The direct opposite of something is its antipode. If your teacher asks what the antipode of the North Pole is, the answer is obvious: it's the South Pole.
Mathematicians and geographers both use the term antipode to describe things that are on opposite sides of a sphere. In geography, this is often referred to as "the antipodes," points that on the earth that are diametrically opposed. In math, an antipode can also be called an "antipodal point," used for two points falling on a line that goes through the middle of a sphere. The Greek root is antipous, "with feet opposite ours."