Take a look at 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 sofa and William is completely correct. Since he was sound asleep on the library sofa is a subordinate clause, so it cannot stand alone. William was oblivious ... begins a main clause. When a subordinate clause introduces a main clause, you need a comma between them.
Go back to the sentence to try again.