Take a look at 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 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 Mrs. Perry, their professor eavesdropped
from inside a far stall.
Removing the comma after professor creates an error. Their professor, 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.
Go back to the sentence to try again.