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 mirror creates an error. Bitterly complaining about the difficult midterm ... is a participle phrase. You need a comma to connect it to the main clause in front.
You might want to review the rules.