Take a look at the original sentence again:
Susan ducked down when she saw Mr. Hayden, her English teacher, walk into
Tito's Taco Palace, unfortunately, a burrito
wasn't big enough to hide behind, so poor Susan had to explain why she had
missed his class again.
You wanted to fix it this way:
Susan ducked down when she saw Mr. Hayden, her English teacher, walk into
Tito's Taco Palace unfortunately, a burrito
wasn't big enough to hide behind, so poor Susan had to explain why she had
missed his class again.
To delete the comma between Palace and unfortunately would cause an equally bad problem, a fused sentence. A fused sentence occurs when you have two complete sentences joined with no punctuation whatsoever. Susan ducked down ... begins a complete sentence. Unfortunately, a burrito wasn't big enough ... starts the next complete sentence. The spot between Palace and unfortunately needs a stronger break than blank space.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.