As I said if the school specifically require you to do this then it should be detailed in their flying order book/operations manual. In which case the only sensible thing for them to do is to make it a requirement that a pilot completes a dual channel crossing therefore making it PU/T. Unfortunately unless the CAA issue some new specific guidleines this argument is going to go on ad infinitum (check the search there are countless threads like this). As far as i can see it is simply a case of who was P1, if the instructor was then you log it dual, if the instructor wasn't you log it P1 and they cannot log it.