Here is the item again:
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 realized that the period after course makes a fragment. As Professor Perry, their instructor, eavesdropped ... is a subordinate clause. When you attach a subordinate clause to the end of a main clause, you usually need no punctuation.
To fix this problem, you chose 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.
You have correctly employed Punctuation Rule 1:
Main Clause + Ø + Subordinate Clause.
Excellent choice!