Tear her skirt

You move your fingers apart, and as you do the fabric on the edge of the teachers skirt suddenly tears apart, giving a split up the already tiny skirt exposing most of her lucious thigh to her student.

His eyes all but bug out of his head, having loved Ms. Burns long legs since the first moment he sat down in her class. He specifically picked a seat at the front of the room just to be able to have an unobstructed view when she would cross and uncross them.

Ms. Burns quickly makes a grab at her skirt trying to pull it closed and you take advantage of this. Using your magic you force her to pull too hard on the fabric and there is a sudden:

RRIIPP

From the other side of her skirt. Effectively tearing it completely from her body leaving her naked from the waist down at the bottom of the stairs in front of a very horny student.