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 Mrs. Perry, their professor,
(C) eavesdropped from inside
a far stall.
You realized that the period after course makes a fragment. As Mrs. Perry, their professor, 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 Mrs. Perry, their professor, eavesdropped
from inside a far stall.
You have correctly employed Punctuation Rule 1:
Main clause + Ø + subordinate clause.
Excellent choice!