Diving at night seems to be one of those things that people either love or hate. I know of divers who absolutely love diving at night and I know of other divers who would have a serious panic attack if they had to go into the water after dark.
A good way to get students accustomed to being in the water at night is to schedule the night dive for when there is a full moon and to start the dive at dusk. The students enter the water and can see fairly well with the available light. Once submerged, the divers become comfortable with the familiar feelings of being underwater. Eventually it becomes dark and most students do not even realize the gradual change from dusk to darkness. Often students are surprised by how well they can see underwater after dark.
A comprehensive dive briefing and orientation helps to calm the more nervous students. Post dive debriefs also helps students process the night dive and enables you to clear up any uncertainties.