Here is the item again:
As all of her classmates were sharing results
(A) Melody quietly left
the room (B)
and found a quiet corner in the hallway
(C) so that she could
see her grade without embarrassing herself in front of her friends.
You correctly realized that a comma after results was needed. To fix the problem, you chose to do this:
As all of her classmates were sharing results,
Melody quietly left the room and found a quiet corner in the hallway
so that she could see her grade without embarrassing herself in front of
her friends.
As all of her classmates were sharing results is a subordinate clause. You have correctly employed Punctuation Rule 2:
Subordinate clause + , + main clause.
Excellent choice!