Medium turns is a good exercise to teach when there's a relatively low cloudbase. If there's a decent horizon, fly it below cloud otherwise fly above 8/8 where there'll be less turbulence and a good horizon.
Whereas for C&D, you really need a good high cloudbase.
Once both exercises have been grasped by the student (the order isn't important), C&D turns can be included as required during subsequent exercises such as stalling.