Take another look at the original sentence:
You wanted to fix it this way:
To delete the comma between clock and there would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two main clauses joined with no punctuation whatsoever. Adriana looked at the clock is the first main clause. There was still a half hour left of class is the second main clause. The spot between clock and there needs a stronger break than blank space.
You might want to consult the rules for fixing comma splices and fused sentences.