While I agree that the above is an important point, IE not having a positive pressure (small) in the mask to push water up the nose, I agree with Nigel the important thing is CONCIOUS control of the soft palate and this is not always easy. Many people even competent surface swimmers have never thought about what goes on at the back of the roof of the mouth, but as Nigel says this is key to control of breathing out or in via nose or mouth. I would suggest an exercise perhaps best performed after away from the pool. P Get student to place hand in front of mouth, exhale through nose then exhale through mouth, check they can tell which is which by feeling on hand. Inhale through nose exhale through mouth three times. Inhale through mouth exhale through nose three times. Then three times try inhale nose exhale mouth, inhale mouth exhale nose, ask if they can do it. Then reverse inhale mouth exhale nose. After that ask them to try to feel what they are doing at the back of their throat to enable them to do this. This hopefully will bring to conscious control what in many people is, like breathing an unconscious series of muscular actions.