Tinny's hit the nail on the head on this one.
The matter needs to be referred to the boss to sort out. That way the decision will be more impartial, and hopefully have the STUDENT'S interests in mind.
Did you fly with the student prior to the request, Apache? If so, perhaps he/she feels more comfortable with you.
Quite often (particularly in busy flying schools) students are allocated to instructors purely based on what is good for the company, and not necessarily the student.
Unfortunately, one of the downfalls of that is the very situation you now find yourself in.
The key here is that the student's welfare is essential to the success of the company, so you should act in their interest and refer the matter to the boss.
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JARSE! Pull and rotate 90 degrees