You do not have to use the microphone. It depends on your classroom situation and noise level. If you do use the microphone, you must do the set-up and test immediately before filming. Do not turn off the iPad or the microphone before filming or you will need to do the set-up procedure again.
Practice often with the microphone before it really counts. If the microphone is not properly functioning, you will have no sound for the video. This cannot be reversed.