Here is the item again:
Since he had fallen asleep on the library sofa, William was unaware that
his twenty-eight algebra classmates were sweating, sighing, and wracking
their brains as they tried to ace their final exam,
poor William, on the other hand, 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 unaware ... begins the first complete sentence. Poor William, on the other hand, 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.