Review the item:
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.
William was unaware is a main clause. A main clause can stand alone as a complete sentence. That his twenty-eight algebra classmates were sweating ... is a subordinate clause.
When a subordinate clause follows a main clause, you will typically not need a comma.