Here is the original item:
Gloria and Scooby stood in front of the bathroom
mirror,
(A) bitterly
complaining about the difficult midterm and stern teaching in their biology
course. (B)
As Professor Perry, their instructor,
(C) eavesdropped
from inside a far stall.
You wanted to do this:
Gloria and Scooby stood in front of the bathroom mirror,
bitterly complaining about the difficult midterm and stern teaching in their
biology course. As Professor Perry, their instructor
eavesdropped from inside a far stall.
Removing the comma after instructor creates an error. Their instructor, an appositive, requires a comma in front and behind. Reconsider your choices. There is a fragment to be found!
You might want to review the rules.