Here is the sentence again:
Since he was sound asleep on the library sofa, William was oblivious to
his twenty-eight algebra classmates, who were sweating, sighing, and wracking
their brains as they tried to ace their final exam,
poor William was quietly earning a zero.
The comma between exam and poor causes a comma splice. At this spot, there are two complete sentences joined with a comma alone. William was oblivious ... begins the first complete sentence. Poor William was quietly earning a zero is the next complete sentence.
To fix this problem, you could put a period after exam and capitalize the P that begins poor.