Follow the Leader
Read Desireé Rumbaugh's response:
One of the most effective skills we all must learn is "taking the seat of the teacher" with grace and power. Students do need to know that you are confident in your ability to conduct the class in an orderly fashion. If you allow a student to do his or her own practice during your class, you dilute the power of your ability to offer something of value to the group.
This student needs to be told directly, in private, that it is not respectful to you or the others to do her own practice during your class. If she's willing to come to the class and demonstrate proper studentship, then she will be welcomed. If not, then it would be better for all concerned that she practice at home. This student, if she is at all developed as a yogi, will understand. If not, then you are doing her a great service by beginning the awakening process around ego.
This may sound daunting at first, if you've been a bit intimidated by this student. But once you take this step, you will see how easy it actually is, and the others in the class will thank you for it.