Standard cartographic practice is to put North at the top, regardless of the hemisphere depicted. In Australia, however, novelty maps are often sold featuring the "correct" orientation, with South at the top. Our Australian Professor of Geography, Reginald Golledge, has one in his office.
While maps are occasionaly printed with other orientations (California, for instance, is often rotated about 20 degrees clockwise to better fit on a rectangular piece of paper), it can be suprisingly difficult to recognize features by their shapes when they are drawn upside down.
As an excercise, try turning a world map upside down and quickly identifying the continents. It is likely to take quite a bit longer than when the map is North up.
This phenomenon is known as "mental rotation", which reflects the hypothesis that the map image must be turned to match a figure in our heads. Research has shown that the more a map is turned, the longer it takes to recognize features.
Sometimes having some other direction than North at the top of the map can be useful. For instance, we often turn a road map when driving to match the orientation of the roads from our point of view. In this case, we are substituting map rotation for mental rotation.
My research has involved designing moving map displays for cars and airplanes. This mode, where the map rotates to match our point of view, is called "track up", or "heading up." Usually, presenting the map to a driver heading up cuts down on the time required for mental rotation, and thus makes the map easier and safer to read. But people vary greatly in their ability to perform mental rotations. It might be interesting to see which of your students can perform these mental rotations easily, and which have more difficulty.
I have seen maps with north down and south up, but I don't think they were from the southern hemisphere. I called Pacific Traveller Supply and the woman I spoke to said they have a couple maps like that, one of the world and one of CA, but they publish them themselves. She did, say, however, that she recently got back from a trip to Peru, and, although she did not see any "upside down" maps while she was there, she did notice that when she showed locals the maps she had (like if she were asking for directions) they oriented them much less consistently than we do here. We are comfortable orienting maps so that N is up, but they would turn them every which way to look at them. I don't know if that is a South American thing or just Peruvian.