Hi,
C) She realized it was her schoolmate.
(E) She realized a girl who had fainted was my classmate.
Regarding sentence c, i guess that "it" is used for only non-living objects.
Can the sentence e be rephrased as "She realized that a girl who had fainted was my classmate" to bring more clarity.
C is fine and clear the way I wrote it. It's standard and correct usage to use 'it' to refer to a person that you are identifying..
eg Q- Who is it? A- It's Tom.
eg A blonde woman came into the room. It was Mary.
Best wishes, Clive