Check your candidates photocard has not expired and check they have their Theory Test Certificate. If these items are not in good order it will not of itself result in the test being cancelled BUT it could result in the examiner taking so much time to check items with DVLA/DVSA that there is not enough time left to conduct the test.
Make sure the vehicles L plates are the correct dimensions. Double check the tyres. Make sure any books are properly stowed and that there are no loose items in the car. Again, items that can be corrected on the day (a spare tyre helps) BUT could result in lost time and the test being cancelled due to insufficient time available to conduct the test.
Whatever happens try to keep calm. Some driving examiners are involved in an official dispute with thier employer. However upset you or your pupil are, remember the examiners action is not aimed at you but at the DVSA.