This is a "use your discretion" answer.

A Master of the boat can safely take the boat out - sail - and bring the boat back safely using good decision making, take prudent actions, and handle the conditions with confidence and competence.

If the instructor is training on advanced techniques then more than likely the student is competent and can be the designated Master with the Instructor being the "consultant" on the boat.

If using CrewMate authentication the Instructor can enter themself as Master and then the student can edit the associated entry afterward to make themself as Master - it will leave the Instructor as Master also.  

Just remember - it all comes down to proper discretion. In general, the student should not be overly zealous to enter Master of the vessel time. Master of the vessel means exactly that - you are experienced, competent, and confident.